Celery requires python3-gi and python3-gst-1.0 even when audio/video not enabled

[Ben Sturmfels]

I've just been testing the installation instructions and noticed that I needed to install python3-gi and python3-gst-1.0 to avoid celery startup failures like:

Apr 29 00:34:00 swing-virgo celery[6374]:   File "/srv/mediagoblin.example.org/mediagoblin/mediagoblin/media_types/video/transcoders.py", line 34, in <module>
Apr 29 00:34:00 swing-virgo celery[6374]:     import gi
Apr 29 00:34:00 swing-virgo celery[6374]: ModuleNotFoundError: No module named 'gi'

and

Apr 29 00:46:38 swing-virgo celery[7111]:   File "/srv/mediagoblin.example.org/mediagoblin/mediagoblin/media_types/video/transcoders.py", line 35, in <module>
Apr 29 00:46:38 swing-virgo celery[7111]:     gi.require_version('Gst', '1.0')
Apr 29 00:46:38 swing-virgo celery[7111]:   File "/usr/lib/python3/dist-packages/gi/__init__.py", line 129, in require_version
Apr 29 00:46:38 swing-virgo celery[7111]:     raise ValueError('Namespace %s not available' % namespace)
Apr 29 00:46:38 swing-virgo celery[7111]: ValueError: Namespace Gst not available

This should only happen if audio/video is enabled. For now I've updated the install docs to include these dependencies.