Create a Bazel auto-roller

In #15, I switched from hardcoding Bazel 1.0.1 in the .travis.yml config to installing Bazel via their published APT repository. This has the advantage that we pick up the latest stable release on every PR, but we lose build reproducibility -- reverting and resending a previous PR can result in an entirely different result if we pick up a new version of Bazel.

We should create create an auto-roller that periodically checks for new Bazel versions, sends a PR to upgrade the linux and macos versions, and lands it if all tests pass.

@cbracken what about in a first step switching to bazelisk? then you would have the option to have the bazel version to a single. The aut-roller could use this file, and having bazelisk would make migrations easier.