gjs now depends on js17

[Colin Walters <walters verbum org>]

A quick heads up: gjs git master now depends on mozjs-17.0 (the new
Spidermonkey standalone release).  jhbuild and gnome-ostree have been
updated.

For GNOME 3.8: gjs has a gnome-3-8 branch which still depends on moz185,
and if you're a distributor who wants to ship GNOME 3.8 with mozjs-17.0,
please send any fixes to the gnome-3-8-js17 branch.

Going forward, getting the polkit patches upstreamed is a priority.  But
hopefully the new garbage collector work will pay off in reduced memory
usage.