Generate a fresh Travis config file using the haskell-ci script

Summary:
The old Travis script still had targets to test against GHC < 8.0,
and wasn't using Cabal `new-build` among other things.

Reviewers: bgamari, RyanGlScott

Reviewed By: RyanGlScott

Differential Revision: https://phabricator.haskell.org/D4681