Upgrade to latest Bazel version

[celskeggs]

We're currently on Bazel 0.26.1. That's more than five months out of date.

The following non-point releases have been released since the version we're using:

• 0.27
• 0.28
• 0.29
• 1.0
• 1.1
• 1.2
• 2.0
• 2.1

This isn't urgent, because we pin bazel now, but should be done at some point in the near future for the sake of not having trouble next time we upgrade all of our dependencies.