sss/travis: testing against ghc 7.6, 7.8

ghc 7.4 is forbidden as per a452522 (#5 and #3)

pertinent links:
  - http://docs.travis-ci.com/user/languages/haskell/
  - https://www.haskell.org/cabal/users-guide/developing-packages.html#package-properties

related to #21