GHC 8.4.1 support

[chrisdone]

Opening PR for CI check.

[chrisdone]

Seems like we can't build Intero on Windows yet because the network package doesn't build on Windows: https://ci.appveyor.com/project/chrisdone/intero/build/1.0.253#L34325

34323    [2 of 9] Compiling Network.Socket.Types ( .stack-work\dist\893fbf7f\build\Network\Socket\Types.hs, .stack-work\dist\893fbf7f\build\Network\Socket\Types.o )
34324    
34325    C:\Users\appveyor\AppData\Local\Temp\1\stack2744\network-2.6.3.4\Network\Socket\Types.hsc:764:16: error:
34326        parse error on input `CALLCONV'
34327        |
34328    764 | foreign import CALLCONV unsafe "ntohs" ntohs :: Word16 -> Word16
34329        |                ^^^^^^^^

[chrisdone]

Opened an issue haskell/network#313 for the network package on Windows.

Otherwise, Windows and OS X both pass the build. Windows isn't going to build until the network package is fixed anyway, so I'm merging this in with GHC 8.4 on AppVeyor commented out.