Enable CI notifications for only the master branch

[matthew-reynolds]

📖 Documentation Error

We would like to be able to send notifications (Slack and/or email) on the status of the build from Travis. However, we would like to notify only when the build status master branch changes. We don't care about feature branches or pull requests, and we don't care if the status stays unchanged. We only care about the status of the master branch.

Looks like neither Travis nor Circle support branch-specific notification filtering at the moment - Travis has a ticket open and says it's WIP (See here), Circle is in a weird spot where it worked in 1.0 but not in 2.0, despite employee claims that old syntax should be valid, so a bit harder to know the status there (See here).

I propose we disable all notifications for now, since they should be a rarity anyways thanks to the restrictions on master branch and the PR status checks. And then once Travis adds this feature, we can enable branch-specific notifications.