gcc: 4.9.1

Version bump gcc from 4.8.x to 4.9.1 on @jacknagel’s
[suggestion](https://github.com/Homebrew/homebrew/pull/30872#issuecommen
t-49511203).

Following the discussion I’ve also killed the ``` HEAD ```, but if
Mike/Jack/Adam/Misty/etc want that back in I’ll submit another commit
to re-insert it.
gcc: update 4.9.1 bottle.

Closes #30970.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>