(RE-2222) Push gems to internal server on release

This commit adjusts the ship_gem utility to include a try for pushing
the completed gem to the stickler host, currently rubygems.delivery.puppetlabs.net.
The host name can be overwritten using the environment variable GEM_HOST.
If it fails it puts an error for user to check if they can access the server and have stickler installed.
While you're in here, would you mind updating this `gem push` to use `ex` also? `%x{}` won't raise an exception in a rake task, which means we could theoretically fail to push the gem but continue on.