Retry fetch specs with `--retry`

This PR adds a general purpose Retry class that can be re-used where-ever retry code is needed!

It also allows you to call `bundle install --retry 3` which will attempt to connect to ruby gems 3 times before failing, and emit a warning message each time
Retry git commands
Merge pull request #2601 from schneems/schneems/retry

Retry fetch specs with `--retry`