stbt tv, selftests: Explicitly use gst-launch-0.10

This works on Ubuntu 12.10, and is also more explicity about the version
of GStreamer we support (GStreamer 1.0 is slightly incompatible with
0.10).

Macports (OS X) and Fedora's GStreamer 0.10 packages install both
`gst-launch` and `gst-launch-0.10`; Ubuntu installs only
`gst-launch-1.0`.
stbt templatematch: Explicitly use bash to fix error on Ubuntu

The "+=" used on line 31 seems to be a bashism. This wasn't noticed on
OS X or Fedora, where the default shell (/bin/sh) is bash. Ubuntu *does*
have bash installed by default at /bin/bash, just not at /bin/sh.

I could also have used posix features instead of "+=", but `stbt-tv` and
`stbt-screenshot` both use bash already so there's no need to sacrifice
bash's convenience.