Bump max mono version to the d15.2 mono. (#1907)

Make.config

@@ -54,7 +54,7 @@ XCODE_DEVELOPER_ROOT=/Applications/Xcode82.app/Contents/Developer
 
 # Minimum Mono version
 MIN_MONO_VERSION=4.8.0.269
-MAX_MONO_VERSION=4.8.99
+MAX_MONO_VERSION=5.0.99
 MIN_MONO_URL=http://bosstoragemirror.blob.core.windows.net/wrench/mono-4.8.0/e5/e51aa0a7043a8185e264d6332ac7bc1f8f87cd39/MonoFramework-MDK-4.8.0.269.macos10.xamarin.universal.pkg
 
 # Minimum Xamarin Studio version