Add a version command

[cceckman]

@jdkaplan and I had exactly the same thought when addressing #51 --

It would be nice to have a command in Pairing Bot that echoes the version deployed (e.g. the SHA of the deployment commit), so we can check that the deploy was successful in an end-to-end manner.

[jdkaplan]

PB has a version command now! It prints the App Engine version (basically a timestamp).

If anyone knows how to get App Engine to build a Go app with -buildvcs=true, that would be even better!

(I'm leaving this open for a little longer in case we do get a way to get commit info threaded through somehow.)