brew.1: reference GCC 4.9.
gcc: provide version_suffix class method.
git-annex: use gcc version_suffix class method.
OS::Mac: allow clearing compiler version cache.

Without this it’s impossible for brew-test-bot to be able to verify if
installing GCC has allowed it to fix a compiler selection failure.
brew-test-bot: clear compiler version cache.

This fix means that brew-test-bot can bottle formulae that require GCC
again.