Release 2015.9

There aren't any major new features, but this release has some notable
bugfixes.  For example, 2015.8 broke xdg-app due to an internal
change in how mainloop contexts were handled for pull - that's now fixed.

Another notable change is a new API `ostree_sysroot_prepare_cleanup()` which
is now used by `ostree admin deploy`.  What this enables is to do:

```
ostree pull repo:$commitid
ostree admin deploy repo:$commitid
```

Before this, `admin deploy` would prune the repository, removing the
un-referenced commit.

In other words, you can now more easily pull the exact (possibly
older) commit you want and deploy it.

New contributors:

 - Leandro Santiago
 - Gatis Paeglis

Bugfixes from:

 - Alexander Larsson
 - Colin Walters
 - Giuseppe Scrivano
 - Matthew Barnes

Git-EVTag-v0-SHA512: d528475ccdc65a882cdda8d2a5f9b1925a599d65b51207e2dace4a4a3ea14a71d5f1dc6803077d1b62b51ac7b03816b249a982d8da074ddbe198accd8476c341
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