script/release-candidate should fail if destination branch is not master

[shawnbot]

As evidenced by #502, it's quite possible to accidentally merge a release branch into dev. I'm out of practice here, but it would be nice to have a guard against this, either as a CI failure or maybe a Probot?

[jonrohan]

This would be too aggressive because there are situations where we might want to merge a branch somewhere else other than master. The approach we're taking instead is to do away with the dev branch #512