gjs: Switch to js24

[Colin Walters <walters verbum org>]

Hi,

Just a heads up that I'd like to land
https://bugzilla.gnome.org/show_bug.cgi?id=711046

Which is Tim's work on porting gjs to mozjs24, the next stable
standalone SpiderMonkey release.  He's been using this branch on Ubuntu
for some time, and I just switched gjs in Continuous to target it:

https://git.gnome.org/browse/gnome-continuous/commit/?id=ccfac3078b38c1f212a0b0504c1d792b44972633

I'll update jhbuild soon for this, and we'll try landing it for 3.12.

At the moment, PolicyKit is not ported, but I'll likely take a look at
doing so this cycle, so that GNOME distributors don't need to ship two
different SpiderMonkey releases.