Add GHC 7.8 testing to Travis config

See https://github.com/hvr/multi-ghc-travis and
http://ro-che.info/articles/2014-03-08-happy-alex-ghc-7.8.html for gory
details.
Merge pull request #611 from maxsnew/travis

Add GHC 7.8 testing to Travis config
Add (failing) equality tests
Fix sum type equality check