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.

Again, people might have to manually `brew uninstall gfortran`.

@wjwwood, @colincsl