Fix exception loading .NET Portable profiles on Mac.

In Mono 3.2.3 on the Mac there are two profiles with the same supported
frameworks:

    Profile7 and ProfileXI7
    net45+win+MonoAndroid10+MonoTouch10

NuGet adds these into a dictionary using the supported frameworks as a key
and expects the target frameworks to be different for all profiles.
Fix json.net failing to install into PCL project on Mac.

Make the compatible PCL profile checks only compare Mono
if both the NuGet package and the local system PCL profiles
include Mono.