title: Project load fails for some projects targeting .Net 4.6 in VS2013
body: <p>I have a solution with a number of C# projects targeting .Net 4.6 and one native C++ project (no CLR). After using the solution with VS2015 I now get errors when trying to open the solution in VS2013 with half the projects failing to load.</p>
  
  <p>The native C++ project and some of the C# projects give the error:</p>
  
  <blockquote>
    <p>error  : A numeric comparison was attempted on "$(TargetPlatformVersion)" that evaluates to "10.0.10069.0" instead of a number, in condition "'$(TargetPlatformVersion)' > '8.0'".  C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets</p>
  </blockquote>
  
  <p>I'm not sure why some projects correctly load and others don't, however, it does look like all C# projects which reference NuGet packages fail.</p>
  
  <p>Is there a way to fix these errors and allow the solution and projects to be opened in both VS2013 and VS2015?</p>