Host a rubygems mirror

1. Prevent problems when gems are pulled (for how long?).
2. Will prevent CI builds failing.
3. Will reduce the load on rubygems.org

Options:

1. https://rubygems.org/gems/rubygems-mirror/
2. https://rubygems.org/gems/gemstash
3. https://rubygems.org/gems/gemirro

Criteria:

1. Faster (likely)
2. Reliable (likely)
3. Works if gem is yanked from rubygems
4. Prevents an existing version from having new code