stbt.py: Avoid deprecation warning with pygobject >= 3.12

See
https://help.gnome.org/misc/release-notes/3.12/developers.html.en#pygobject

Note that `gi.version_info` was added in 3.3.5, but our supported
distros that ship GStreamer 1 also ship a python-gi newer than 3.3.5:

* Fedora 19 ships 3.8.3 (Rawhide --the future Fedora 21-- will ship
  3.12).
* Ubuntu 12.10 ships 3.4.0 (14.04 will ship 3.12).
* Debian testing ships 3.10.

However Ubuntu 12.04, which runs our self-tests (via travis-ci) with
GStreamer 1.x from the gstreamer-developers ppa, still has an older
python-gi so we need to use `getattr` with a default value.