CI: Don't build branch and PR on push
CI: Don't build branch and PR on push

When you push to a branch that has a PR open Travis-CI will now perform
two builds, one for the branch and one for the PR:
https://docs.travis-ci.com/user/pull-requests/

> If you see two build status icons on your GitHub pull request, it
> means there is one build for the branch, and one build for the pull
> request itself (actually the build for the merge of the head branch
> with the base branch specified in the pull request).

This is a bit annoying and a waste of resources since we're running
the test suite twice.

Instead, this limits push builds to the master branch. PRs will still
be built b/c we have Travis-CI configured to "Build pushed pull
requests" in the CI settings for this repo.

pion/dtls