Could not load file or assembly 'WebGrease, Version=1.5.1.25624, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies

I know it's kind of late for the OP but I ran into the same problem while trying out the Bootstrap 3 for MVC 4 NuGet package, in my case it had something to do with the Microsoft.AspNet.Web.Optimization package, and managed to find a simple solution.

Try executing the following commands in the package manager console:

Install-Package Microsoft.AspNet.Web.Optimization
Update-Package WebGrease
Uninstall-Package Microsoft.AspNet.Web.Optimization
Uninstall-Package WebGrease
Install-Package Microsoft.AspNet.Web.Optimization
Update-Package WebGrease

The first two lines had no effect for me since those packages were already installed and updated by the Bootstrap 3 for MVC 4 package, but I ran them anyway and then it all compiled and ran great.

This error is because Microsoft.AspNet.Web.Optimization 1.1.3 internally references WebGrease 1.5.1.25624 even though the Nuget package, itself, has a dependency on WebGrease 1.5.2.14234. Someone clearly messed up while creating the Nuget package.

To solve this, add this assembly binding in your Web.Config.

  <dependentAssembly>
    <assemblyIdentity name="WebGrease" publicKeyToken="31bf3856ad364e35" culture="neutral" />
    <bindingRedirect oldVersion="0.0.0.0-1.5.1.25624" newVersion="1.5.2.14234" />
  </dependentAssembly>