Could not load file or assembly 'Microsoft.CodeAnalysis, version= 1.3.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies

I can't get the light bulb in Visual Studio 2015, and when I go to the context menu, I now get an error message:

Could not load file or assembly 'Microsoft.CodeAnalysis, version= 1.3.1.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

So, for me, the answer was to install Visual Studio Update 3, found in Visual Studio under
Tools >> Extensions and Updates...