Travis CI: Don't install gstreamer0.10-ffmpeg or -plugins-bad

stb-tester and its unit tests don't require this; it's only necessary if
the user's "source pipeline" uses ffmpeg GStreamer elements.

We do need gstreamer0.10-plugins-base for videotestsrc etc (presumably
this was an implicit dependency of -plugins-base or -ffmpeg).
Travis CI: Remove GStreamer dev packages

We no longer build GStreamer plugins (since a35848f).