Add unique constraint to Builds.ref

- 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  since that was superceded by build_pull_requests option on Repository.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
Merge remote-tracking branch 'public/rob/unique-index-builds'