Watch how to fix it on YouTube:

 Some NuGet packages were installed using a target framework different from the current target framework and may need to be reinstalled. Visit http://docs.nuget.org/docs/workflows/reinstalling-packages for more information.  Packages affected: EntityFramework, Microsoft.Net.Http

- in my case I got this error because I changed target framework to .Net 4.0 (from .Net 4.5)
- to fix it - go to
Tools/NuGet Package Manager/NuGet Package Console and execute the following command:
PM> Update-Package -reinstall