Support only GHC 7.6 for the time being.

This commit addresses a build issue with GHC 7.4 by simply ignoring
it. We can address broader compiler support later.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #11 from blt/blt-only_76

Support only GHC 7.6 for the time being.