Restrict CI build to master branch and pull requests.

I noticed two issues with our CI setup:

- For core team members working directly off rackerlabs/canon, a build was being triggered twice for each PR: one build for the branch, and one build for the PR.
- For other people working off of forks, their builds were failing because of encrypted credentials in our Travis configuration.

This should definitely fix issues number one. I hope it fixes the second issue.