gcc49 4.9.1

Update to latest stable release, use HTTPS on homepage URL, change, single quotes to double, change dependencies to the regular formulas instead of particular versions.
Assuming we keep gcc49 in the *homebrew-versions* repo, is it necessary for this to only depend on versioned formulas? This compiles fine against the current formulas of all of its dependencies (gmp 6.0.0a, libmpc 1.0.2, mpfr 3.1.2-p8, isl 0.12.2), and there aren't version formulas for all of those releases.