rosdep rule: change gfortran on osx/homebrew gcc

After recent changes to the homebrew-provided gcc formula to make all binaries version-suffixed, but retain an additional suffix-less gfortran binary we can rely on this formula again and do not need to maintain our own gfortran formula.

This reverts #4607; see also 328#issuecomment-45648207.
Merge pull request #4669 from NikolausDemmel/patch-2

rosdep rule: change gfortran on osx/homebrew gcc