[rust-dev] Travis CI is building Pull Requests

Greetings Rustafarians! As of a few minutes ago, along with the merging of #12437, I would like to inform everyone that we're now going to be building all Pull Requests on Travis CI [1]. Travis is a continuous integration system which integrates very well with Github and makes it fairly easy to run lots of builds. Running a full "make check" would be quite burdensome for Travis, so we're limiting it to building a stage1 rustc and then running some of the broader, yet quick test suites. These builds should take about 20-30 minutes per build, and builds will be triggered for each new PR along with each force-push to the PR. We will continue to gate all PRs on bors, and bors will continue to run the exhaustive test suites on our "tier 1" platforms. Using Travis will hopefully provide quicker feedback about misformatted files, failed tests, typos, etc. It is still highly recommended to run "make check" before submitting a PR (as always), but Travis will hopefully start serving as a first line of defense for bors. The ultimate goal is for this to take some load off bors with fewer failed PRs. You can check the status of a PR by looking at the bottom of the page on Github, or visiting Travis's status page [2]. That's all for now, keep on being awesome everyone! [1] - https://travis-ci.org/ [2] - https://travis-ci.org/mozilla/rust