Bumping version to 0.4.2.

* For some reason this commit never got pushed as part of master to GitHub, and became detached.
* Instead of rebasing master, I'm recreating the commit and re-tagging v0.4.2.
* I'm not republishing v0.4.2 to npm, because changes since 0.4.2 have been released don't affect functionality.
* Whoops!