cayley 0.4.0

Release notes: [Bolt, Speedups and Fixes](https://github.com/google/cayley/releases/tag/v0.4.0)

> Worth noting: you may have to reload your data in this version, as the database format has changed (hence the minor version bump). This is part of working with early software.

> On the plus side, you'll notice quite a few benefits, including the new features.
Seems we should have it either in `post_install` or omit the caveats.