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.
Sorry, should have said before but you need to remove `revision 1` on version bumps.