Reference hint fix for Json.NET.

This reference was causing a build error trying to find the NuGet-restored DLL on a fresh copy of the repo. I just lined up the hint path with the current location where Xamarin Studio restores NuGet packages.
Merge pull request #53 from patridge/patch-1

[F#/iOS] Reference hint fix for Json.NET.
Merge remote-tracking branch 'origin/master' into fix-fsharp-build

Conflicts:
	F#/XamarinStore.iOS/XamarinStore.fsproj