update GHC versions used on travis
update GHC versions used on travis

I saw that (nightly) travis builds are failing because we now only support 8.4 or higher to boot a GHC HEAD build. I don't know how to achieve this for the OS X section though.
sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc