Cancel Previous travis builds on each push

[mgwidmann]

On each push, kick off a Task to contact the travis build for that old SHA and cancel the job.

Travis' API for builds: https://docs.travis-ci.com/api/#builds

Should be able to add a new function SlackCoder.BuildSystem.cancel/1 taking in the old PR before update and uses build_id/1 https://github.com/mgwidmann/slack_coder/blob/master/lib/slack_coder/build_system.ex#L68 to get the build ID and issue the request to cancel the job.