(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.
This is probably just a nitpick, but can you rename this to reflect that it's different from the upstream public rubygems? E.g., something like `internal_gem_host`?