Add CI for Bazel

[ejona86]

While users are good at noticing when Bazel breaks, I suspect they'd appreciate it to compile without their intervention.

This probably should be on Kokoro.

[drigz]

FYI: https://github.com/bazelbuild/continuous-integration/blob/master/docs/owner.md is the documentation for adding a new project to https://ci.bazel.io.