Dependency-injection Exception when configuring EntityFramework Core

//the Startup.cs file configuration settings.
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();
    services.AddEntityFramework().AddSqlServer().AddDbContext<PortContext>();
}

project.json

 {
  "dependencies": {
  "Microsoft.NETCore.App": {
  "version": "1.0.0",
  "type": "platform"
 },
  "Microsoft.EntityFrameworkCore.Design": {
  "version": "1.0.0-*",
  "type": "build"
 },
  "Microsoft.AspNetCore.Diagnostics": "1.0.0",
  "Microsoft.AspNetCore.Mvc": "1.0.0",
  "Microsoft.AspNetCore.Server.IISIntegration": "1.0.0",
  "Microsoft.AspNetCore.Server.Kestrel": "1.0.0",
  "Microsoft.Extensions.Logging.Console": "1.0.0",
  "Microsoft.AspNetCore.StaticFiles": "1.0.0",
  "MailKit": "1.3.0-beta7",
  "Microsoft.Extensions.Configuration.Json": "1.0.0",
  "EntityFramework.Core": "7.0.0-rc1-final",
  "EntityFramework.MicrosoftSqlServer": "7.0.0-rc1-final",
  "mongocsharpdriver": "2.3.0-rc1",
  "Microsoft.EntityFrameworkCore.Tools": "1.0.0-preview2-final"
 },

"tools": {
  "Microsoft.AspNetCore.Server.IISIntegration.Tools": "1.0.0-preview2-final",
  "Microsoft.AspNetCore.Razor.Tools": {
  "version": "1.0.0-preview1-final",
  "imports": "portable-net45+win8+dnxcore50"
},
  "Microsoft.EntityFrameworkCore.Tools": "1.0.0-*"
},
"commands": {
"ef": "EntityFramework.Commands"
},
"frameworks": {
"netcoreapp1.0": {
  "imports": [
    "dotnet5.6",
    "portable-net45+win8"
  ]
 }
},

 "buildOptions": {
 "emitEntryPoint": true,
 "preserveCompilationContext": true
},

"runtimeOptions": {
 "configProperties": {
   "System.GC.Server": true
 }
},

"publishOptions": {
 "include": [
   "wwwroot",
   "web.config"
 ]
},

"scripts": {
"postpublish": [ "dotnet publish-iis --publish-folder %publish:OutputPath% --framework %publish:FullTargetFramework%" ]
 }
}

    And my PortContext.cs file's OnConfiguring method is given below.

    protected override void OnConfiguring(DbContextOptionsBuilder options)
    {
        var connString =Startup.Configuration["Data:PortContextConnection"];
        options.UseSqlServer(connString);
        base.OnConfiguring(options);
    }

When I run dotnet ef migrations add command I am getting the exception:

An error occurred while calling method 'ConfigureServices' on startup class 'WebApplication8.Startup'. Consider using IDbContextFactory to override the initialization of the DbContext at design-time. Error: Could not load type 'Microsoft.Extensions.DependencyInjection.ServiceCollectionExtensions' from assembly 'Microsoft.Extensions.DependencyInjection.Abstractions, Version=1.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.

Can anyone help me solve this issue?