Build a .xz file on release instead of a .bzip2 file

Previously when we released a .bzip2 file the Gnome release scripts
will just convert this to a .xz file and generate its own checksum.
The .bzip2 is never actually available on the FTP server. This isn't
ideal because we were listing the checksum of the .bzip2 file in the
release message but that is useless information. This patch changes it
to generate a .xz file instead which is also what Clutter does.

Reviewed-by: Robert Bragg <robert@linux.intel.com>