travis: Drop ghc 7.10.3 and use ghc 8.4.4
travis: Drop ghc 7.10.3 and use ghc 8.4.4

Since this project uses some of the new dependent types features of GHC
keeping it working with multiple GHC versions is going to be difficult.
Drop 7.10.3 because it has already stoppped working.

HuwCampbell/grenade,HuwCampbell/grenade