Unique index on Build ref column

This is being added to prevent the race condition of multiple build requests for the same sha.

- constrain the ref column to 40 characters (the length of a SHA-1)
- Many changes to tests to account for new constraint in test data
- Removed undocumented '!buildme' feature from PullRequests controller since that was superceded by build_pull_requests option on Repository.
- add pry-byebug to Gemfile

To: @cheister @square/kochiku-contributors