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  since that was superceded by build_pull_requests option on Repository.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'
Remove promote_branch! method from RemoteServer classes

should not be needed anymore
Make the promote method in BuildStrategy private

* Rename promote to update_branch
Step 1 of server aliases support
Move the remove_server classes to lib/
Fix the favicon path

Rails seems to be using /images/favicon.ico instead of the root now.
Add host and namespace columns to Repository + more

* Renamed Repository repository_name column to just name
* Restructure the remote server classes
* Deprecated the url column on Repository. Soon we will always be
  constructing the url based on the host, namespace, and name fields.
Move migration version to a more recent date