Test multiple GHC versions on Travis

Travis configuration based on https://github.com/hvr/multi-ghc-travis.
Disable travis build on GHC 6.12
Install happy and alex in travis
Import Monoid class for older GHC
Only enable tests in travis for GHC >= 7.2

Some dependencies of test-framework can't be built with older GHCs.
Remove hlint check from travis
Fix install command
Enable builds with multiple GHC versions on Travis