Drop GHC 6 ?

[andreasabel]

The cabal file contains branches for GHC 6:

zlib/zlib.cabal

Lines 87 to 91 in ee937e3

	if impl(ghc < 7)
	default-language: Haskell98
	default-extensions: PatternGuards
	else
	default-language: Haskell2010

Are these tested regularly and if yes, how?
Should we rather drop GHC 6 and stick to GHC >= 7 which is tested by CI?

[Bodigrim]

Thanks, fixed in 8d6961c.