Update version extraction for _version.py to work with the new tag syntax.

The version tag used to be in the form "bup-<version>".

The syntax is now just "<version>" to allow github to produce
reasonable archives (i.e. not bup-bup-VERSION).

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Reviewed-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>