Steps to Reproduce

1. Create F# library
2. Add <LangVersion>6.0</LangVersion> to fsproj
3. Build
4. See FSC: Error FS0246: Unrecognized value '6.0' for --langversion use --langversion:? for complete list (FS0246)

Current Behavior

F# 6 does not build

Expected Behavior

F# 6 does build

On which platforms did you notice this

macOS

Version Used:
monoframework-mdk-6.12.0.158.macos10.xamarin.universal.pkg