Fallback to "modern" mtouch root if MONOTOUCH_ROOT is not set. (#52)

After `12.14.0.114` there is no more `/Developer/MonoTouch` dir dotnet/macios@c669116

Author: viniciusjarina

Touch.Server/Main.cs

@@ -198,7 +198,7 @@ public static int Main (string[] args)
 
 			string mt_root = Environment.GetEnvironmentVariable ("MONOTOUCH_ROOT");
 			if (String.IsNullOrEmpty (mt_root))
-				mt_root = "/Developer/MonoTouch";
+				mt_root = "/Library/Frameworks/Xamarin.iOS.framework/Versions/Current";
 
 			string mtouch = Path.Combine (mt_root, "bin", "mtouch");
 			if (!File.Exists (mtouch))