build: require GHC 7.6 for bootstrapping

Summary:
Per the usual standards, a build of GHC is only compileable
by the last two releases (e.g. 7.8 only by 7.4 and 7.6). To make sure
we don't get suckered into supporting older compilers, let's remove
this support now.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan:
Try to bootstrap with GHC 7.4, watch it fail. Bootstrap
with 7.6 or better, and everything works.

Reviewers: hvr

Reviewed By: hvr

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D167