Fixing Mono & Travis CI Build/Test

Fixing the mono builds by updating Nuget to 1.8.1, and updating NuGet.targets file.
The newer version is more compatible with mono/Xamarin Studio, and doesn't error out.

Also enabling package restore, since it seems like a good idea. 
Fixes issue #197. 
Ooh, good catch, let me see what's going on there.