CI: Don't fail on MacOS if brew cannot remove libtool.
CI: Don't fail on MacOS if brew cannot remove libtool.

This is the better option, as it won't break the CI script if travis
ends up removing the preinstalled libtool bundle for whatever reason.

MrSmile/libass,astiob/libass,xbmc/libass,astiob/libass,Akemi/libass,Akemi/libass,Akemi/libass,ShiftMediaProject/libass,MrSmile/libass,ShiftMediaProject/libass,libass/libass,libass/libass,grigorig/libass,xbmc/libass,grigorig/libass