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

Note that `gi.version_info` was added in 3.3.5. All the distros (that we
support) that ship GStreamer 1.x 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 the `hasattr(gi, "version_info")` check.