[MSBuild] Fix horribly broken ToolsVersion lookups
[MSBuild] Use ToolsVersion 4.0 when 12.0 requested

Mono doesn't support 12.0 yet.
[MSBuild] Ignore VS2013 build test, xbuild doesn't support it
[MSBuild] Don't create projects with 12.0 toolsversion yet

xbuild doesn't support it