Revert "travis.yml: don't build with GHC 7.4"
Revert "travis.yml: don't build with GHC 7.4"

This reverts commit 6913400f5ae76cfaf6195114d5efd0aba28778df.

sopvop/cabal,themoritz/cabal,mydaum/cabal,mydaum/cabal,themoritz/cabal,sopvop/cabal,sopvop/cabal,themoritz/cabal