Newer NuGet.targets version to allow compilation with XBuild on Mono

The version currently in use throws an exception while compiling on OSX under Xamarin Studio. The bug was filed under https://bugzilla.xamarin.com/show_bug.cgi?id=20915.
There I was pointed to a newer targets version in use in the NuGet project. I copied that and verified that compilation of the supported projects now works.

Please check whether it still works fine on Windows (I'd think so, but I cannot verify)