Make RestSharp.Portable project loadable in Xamarin Studio.

When loading the RestSharp.Portable project in Xamarin Studio v4.2 and
above you will get a 'Error while trying to load the project: Unknown
ToolsVersion 12.0'. The fix is trivial, for reference see:

http://forums.xamarin.com/discussion/10460/any-workaround-for-this-error-unknown-toolsversion-12-0