wmflib: add os_version() & requires_os()

Since we're going to introduce Debian Jessie to our production
environment at some point, ubuntu_version() & requires_ubuntu should be
replaced with something more generic. So add os_version(), which has a
similar syntax, but which can handle compound predicates like:

    if os_version('ubuntu >= trusty || debian >= jessie') {
        ...
    }

Change-Id: I2382035ff90c7c4423c5e2b2130a9c0de6c8c707