Add bundler gem packaging tasks to Rakefile

This makes it ever-so-slightly easier to build a new gem.

Change-Id: I70b3344ec722321e9369af55e8dc6e204c89a7b4
Reviewed-on: http://gerrit.causes.com/45308
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>