travis-ci osx: make sure build-tools are always installed
travis-ci osx: make sure build-tools are always installed

It seems that sometimes build-tools packages will not be installed unless
the --all flag is specified.

f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver