Attempt to merge master before running tests

To ensure we don't miss out on updates in master, and therefore merge
broken code, we should try to merge master into the current branch
before running the tests.

This is based on alphagov/specialist-publisher#118
It means abort the merge — my intent was for branch builds to still provide useful feedback for developers working on a branch that doesn't cleanly merge. Github won't have a merge button for branches that don't cleanly merge so we don't have to worry about bad PRs being merged by accident.