handle new `Rollback` info

[pivotal story](https://www.pivotaltracker.com/n/projects/488191/stories/64898846)
Merge pull request #24 from bitzesty/rollback-history

handle new `Rollback` info
Update airbrake
Merge pull request #25 from alphagov/airbreak

Update airbrake
update gem lockfile
default rollback's datepicker to today
Merge pull request #26 from bitzesty/rollback-datepicker

default rollback's datepicker to today
Add guidance for contributors