rpm spec file: Fix dependencies for Fedora 19

On Fedora 19 python-gstreamer1 doesn't exist. We don't actually need it;
it provides overrides to the auto-generated GObject Introspection python
bindings, but nothing that's actually useful to us (we maintain our own
wrapper in gst_hacks.py for now).

We require pygobject3 explicitly (previously it was implied by
python-gstreamer1).

While I'm here, we don't need pygtk2 since the port to GStreamer 1
(commit a071a9cc).