[telepathy-gabble] Update to upstream 0.18.3

This fixes some crash bugs and other issues.

```
telepathy-gabble 0.18.3 (2014-05-07)
====================================

Fixes:

• fd.o #76465: fix a crash in Jingle (Guillaume)

telepathy-gabble 0.18.2 (2014-03-20)
====================================

The “mithril boxing gloves” release.

Fixes:

• update Wocky:
  · don't try to cancel a source ID twice, which issues a critical warning
    in GLib 2.39
  · fix failure to build with recent (Markdown-based) gtk-doc

• plugin loader: don't crash if g_dir_open() fails twice (fd.o #66085, Will)

• fix a crash in 1-1 Tubes on 64-bit machines (fd.o #70038, Guillaume)

• fix enum cast warnings under clang (fd.o #70038, Guillaume)

• add a regression test for fd.o #68829 (Simon)
```