main: Add an explicit --version argument.

Use an explicit `version` action instead of the undocumented (and
deprecated) `version` keyword to ArgumentParser [1,2].

[1]: http://docs.python.org/3.3/library/argparse.html#action
[2]: http://docs.python.org/3.3/library/argparse.html#upgrading-optparse-code

Signed-off-by: W. Trevor King <wking@tremily.us>