gfortran was removed from homebrew, but now is in the ros tap

After discussing this with upstream (homebrew/homebrew) we will be including a copy of the gfortran formula with our ros tap. We discussed this on the rosdep issue tracker: https://github.com/ros-infrastructure/rosdep/issues/328

So this pull request will basically be reverting this pull request: https://github.com/ros/rosdistro/pull/4443

@NikolausDemmel