[ Issue 1395 ] Fixed

Replaced GHC 7.8.4 by GHC 7.8.3 in .cabal tested-with field.

Travis supports GHC via the Haskell platform. ​At the moment, there
isn't a version of Haskell platform including GHC 7.8.4.

I kept GHC 7.8.4 in the hTags program because this program isn't used
in the Travis build.