fixup! Port to Gstreamer 1.0

This patch is the result of running `make` after the changes to stbt.py
in "Port to Gstreamer 1.0".
Don't skip test_that_video_index_is_written_on_eos on OS X

GStreamer 1.0's webmmux (from macports gstreamer1-gst-plugins-good 1.2.3
package) does work correctly on OS X.
(FAILING TEST) Don't skip test_save_video on OS X

GStreamer 1.0's webmmux does work correctly on OS X (see the previous
commit). However, test_save_video hangs indefinitely. I need to
investigate, but in the meantime I shouldn't hide the problem from
myself (or anyone else testing on OS X). test_save_video does pass on
Linux.
.travis.yml: Fix "ImportError: No module named gi.repository"

gir1.2-gstreamer-1.0 installs "GObject introspection data" that isn't
specific to python. We do need to explicitly install python-gobject for
the "GObject introspection" python bindings.