Canonicalize MSBuild ToolsVersion to 12.0

Right now, there's a mix of ToolsVersion 4.0 and 12.0 in the projects.
We should make sure that it's 12.0 everywhere.
Merge pull request #305 from akoeplinger/fix-msbuild-toolsversion

Canonicalize MSBuild ToolsVersion to 12.0