[travis] fix GHC 7.4 by using older transformers
[tavis] unregister happy deps after building it

Happy depends on `mtl` and `transformers`. Those are not needed for the executable to run, so we can safely unregister both of them after building `happy`. This should avoid dependency hell.
[travis] remove cabal constraints try
ghc-pkg only unregisters one package at a time
[travis] cabal: enable unlimited backtracking
[travis] more cabal verbosity
Add transformers bound
Fix build error with cabal 1.16.
Merge pull-request #1.
Fix cabal 1.16 build error.