Revert "Fix tests for Cabal <= 1.16"

The problem the reverted commit tried to fix seems to have been just a
symptom of another root cause that was later fixed as the problem does
not seem to occur anymore even without this changeset also this
changeset broke cabal sandboxes with ghc < 7.8.

Fix #262

This reverts commit 8741323f5b9c5450c40205d0632f61a4b7e42125.

Conflicts:
	Language/Haskell/GhcMod/GhcPkg.hs
Add test for sandbox compatibility with ghc < 7.8