Startup configuration {get set} issue

I have the startup.cs as

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNet.Builder;
using Microsoft.AspNet.Diagnostics;
using Microsoft.AspNet.Hosting;
using Microsoft.AspNet.Http;
using Microsoft.AspNet.Routing;
using Microsoft.Extensions.PlatformAbstractions;
using Microsoft.Framework.Configuration;
using Microsoft.Framework.DependencyInjection;
using Microsoft.Framework.Logging;
using Microsoft.Framework.Logging.Console;
using Microsoft.Extensions;
using OpsTools.DataAccess;

namespace OpsTools
{
    public class Startup
    {
        public Startup(IHostingEnvironment env, IApplicationEnvironment appEnv)
        {
            // Setup configuration sources.
            var builder = new ConfigurationBuilder()
                .SetBasePath(appEnv.ApplicationBasePath)
                .AddJsonFile("config.json")
                .AddEnvironmentVariables();
            Configuration = builder.Build();
            var test = Configuration["ASPNET_ENV: Development"];
        }

        public static IConfiguration Configuration { get; set; }

        // This method gets called by the runtime.
        public void ConfigureServices(IServiceCollection services)
        {
            // Add MVC services to the services container.
            services.AddMvc();
        }

        // Configure is called after ConfigureServices is called.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            loggerFactory.MinimumLevel = LogLevel.Information;
            loggerFactory.AddConsole();

            // Configure the HTTP request pipeline.

            // Add the following to the request pipeline only in development environment.
            if (env.IsDevelopment())
            {
                app.UseBrowserLink();
                app.UseDeveloperExceptionPage();
                //app.UseErrorPage(); 
                //Configuration.Set("AuthConfig","auth.debug.config");
                //Configuration.Set("installFolder", "");
            }
            else
            {
                // Add Error handling middleware which catches all application specific errors and
                // send the request to the following path or controller action.
                app.UseExceptionHandler("/Home/Error");
               // Configuration.Set("AuthConfig", "auth.config");
            }

            // Add static files to the request pipeline.
            app.UseStaticFiles();

            // Add MVC to the request pipeline.
            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
                // Uncomment the following line to add a route for porting Web API 2 controllers.
                // routes.MapWebApiRoute("DefaultApi", "api/{controller}/{id?}");
            });
        }
    }
}

In another class

 public class AniRepository
    {
        private string _dbPrefix;
        private string _connectionString;

        public AniRepository(string connectionString)
        {
            _connectionString = connectionString;
            _dbPrefix = Startup.Configuration.Get("Database:DbPrefix");
        }

I get the error 

Error	CS1061	'IConfiguration' does not contain a definition for 'Get' and no extension method 'Get' accepting a first argument of type 'IConfiguration' could be found (are you missing a using directive or an assembly reference?)	

My project.json file

{"webroot": "wwwroot","version": "1.0.0-rc1-update","dependencies": {"Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final","Microsoft.AspNet.IISPlatformHandler": "1.0.0-rc1-final","Microsoft.AspNet.Diagnostics": "1.0.0-rc1-final","Microsoft.AspNet.Mvc": "6.0.0-rc1-final","Microsoft.Extensions.Logging.Console": "1.0.0-rc1-final","Microsoft.VisualStudio.Web.BrowserLink.Loader": "14.0.0-rc1-final","Microsoft.AspNet.StaticFiles": "1.0.0-rc1-final","Microsoft.Extensions.Configuration": "1.0.0-rc1-final","EntityFramework": "6.1.3","jqGridWebApi": "1.1.4","EnyimMemcached": "2.13.0","xunit": "2.2.0-beta1-build3239","Moq": "4.2.1510.2205","NLog": "2.1.0","CryptSharpOfficial": "2.1.0","System.Linq.Dynamic": "1.0.4"
  },"commands": {"web": "Microsoft.AspNet.Hosting --server Microsoft.AspNet.Server.WebListener --server.urls http://localhost:5001","kestrel": "Microsoft.AspNet.Hosting --server Kestrel --server.urls http://localhost:5005","ef": "EntityFramework.Commands"
  },"frameworks": {"dnx451": {"frameworkAssemblies": {"System.configuration": "4.0.0.0","System.Data": "4.0.0.0"
      },"dnxcore50": { }
    }
  },"exclude": ["wwwroot","node_modules","bower_components"
  ],"publishExclude": ["node_modules","bower_components","**.xproj","**.user","**.vspscc"
  ],"scripts": {"prepublish": [ "npm install", "bower install", "gulp clean", "gulp min" ]
  },"configurations": {"Staging": { }
  }
}

Anyway I want to use "1.0.0-rc1-update".