Assumes default branch is 'master'

Created by: alokpr

When rebase fails, marge-bot attempts to checkout the default branch, which it assumes is named 'master': https://github.com/smarkets/marge-bot/blob/master/marge/job.py#L348

This fails on repositories without a 'master' branch leading to uncaught exception. I think there should be a config setting for default branch name.