Make ghcjs 8.2 vs ghcjs 8.4 explicit
Make ghcjs 8.2 vs ghcjs 8.4 explicit

We want to make it mirror how ghc is handled. The ghc 8.4 based ghcjs
is used by defaault for now.

reflex-frp/reflex-platform,reflex-frp/reflex-platform,ryantrinkle/try-reflex,reflex-frp/reflex-platform