What this PR does

Clean up MEF log when extension installed in Visual Studio 2017

• Use TargetVersion="[14.0,15.0)" to exclude GitHub.TeamFoundation.14
  MEF component from VS2017

The source.extension.vsixmanifest now includes this:

    <Asset Type="Microsoft.VisualStudio.MefComponent" d:Source="Project" 
        d:ProjectName="GitHub.TeamFoundation.14"
        TargetVersion="[14.0,15.0)" Path="|GitHub.TeamFoundation.14|" />
    <Asset Type="Microsoft.VisualStudio.MefComponent" 
        d:Source="Project" d:ProjectName="GitHub.TeamFoundation.15"
        TargetVersion="[15.0,16.0)" Path="|GitHub.TeamFoundation.15|" />

Unfortunately the TargetVersion="[15.0,16.0)" won't be understood by Visual Studio 2015 (so its MEF log will remain dirty).

How to test

Install in Visual Studio 2017 and use the Clear MEF Component Cache extension to Show MEF Errors (it's on the Tools menu). This should now be empty in Visual Studio 2017.

Errors will still appear in Visual Studio 2015 (it doesn't support the TargetVersion attribute).

Fixes #1704