Canonicalize MSBuild ToolsVersion to 12.0

This is similar to https://github.com/dotnet/corefx/pull/305, i.e. to get rid of the mix of ToolsVersion 4.0 and 12.0 in the projects.

Note that this is not strictly required for my Mono work. However, it allows us to not load the Microsoft.Build.Utilities.v4.0.dll (which is probably just a nitpick :smile:)

before:
![before](https://cloud.githubusercontent.com/assets/1376924/5554868/a84c1d8a-8c7d-11e4-8631-23dbc0550658.png)
after:
![after](https://cloud.githubusercontent.com/assets/1376924/5554869/a85634be-8c7d-11e4-8780-cfaf7197c213.png)

Looks like there is already a mis-match with the M.B.F being 12, so I'm happy with this change. Should we also explicitly put Version=12.0.0.0 on the M.B.Utilities.v12 as well?