fixes fsharp/FAKE#165 for 32-bit machines
"On 32-bit machines they can be found in: C:\Program Files\MSBuild\12.0\bin"
http://blogs.msdn.com/b/visualstudio/archive/2013/07/24/msbuild-is-now-part-of-visual-studio.aspx
Merge pull request #205 from ctaggart/issue165win32

fixes fsharp/FAKE#165 for 32-bit machines