Refactor how we fetch gems and extra files.

This should cut down the amoutn of time we spend in Docker before
bringing up Crowbar, and have less churn in the future.

This also pulls in an updated Docker image.
Merge pull request #178 from VictorLowther/refactor-gem-fetching-at-bootstrap

+2 This resolves the issue from https://groups.google.com/forum/#!topic/opencrowbar/Ogp0x98JRQo 
Refactor how we fetch gems and extra files.