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)

Since the assembly name is already suffixed with v12 it wouldn't make a difference (compared to the other assemblies) other than consistency.

Do you want me to add it?