See #2224 for context

It seems at some point I (accidentally?) bumped the crate's prerelease
version from v0.3.0-pre.2 to v0.4.0-pre.0. We've never had a v0.3 stable
release.

I'm dropping the version down to v0.3.0-rc*, and will yank the existing
v0.4.0-* prereleases.