Decouple Error Prone version from Bazel version

Bazel 0.4.5 upgraded Error Prone, and in so doing introduced new build failures that weren't there in Bazel 0.4.4. We weren't able to upgrade to Bazel 0.4.5 until we addressed all of the new Error Prone failures. The same thing happened with Bazel 0.4.4.

Those Error Prone issues are valuable, but I wish we could pin our version of Error Prone to an older version while upgrading to Bazel 0.4.5, so we can get the benefits of the new version of Bazel without fixing all of the Error Prone issues.

Start by tracing how Bazel selects and upgrades its Error Prone dependency around the Bazel 0.4.4 and 0.4.5 changes. Determine where the version is coupled, then verify that an older Error Prone version can be pinned while upgrading Bazel without introducing the newer Error Prone failures.

Written by the indexing model from the issue text.