Fixing path to MSBuild for VS2013.

Since VS2013 it ships with its MSBuild so using the .Net version MSBuild causes problems with  build targets based off its build number. eg it could not find the right version of  "WebApplications.target" with vs 2013 install. Reference http://blogs.msdn.com/b/visualstudio/archive/2013/07/24/msbuild-is-now-part-of-visual-studio.aspx.

I also notice that msbuild path is defined in both files watcher_dot_net.rb and watcher_dot_net.rb, I am afraid I have very little ruby experience did not feel confident removing the duplication.