Prepare release 0.4.2

These are my proposed changes for finalizing the 0.4.2 gem. If they look good, you can `rake release` to build and push the gem, or just do it manually if you prefer. Or add me to the gem (`gem owner GEM -a EMAIL`) and I will do it.

The `rake release` task will also generate an empty commit and ask you for a commit message. I like to use this for the change summary (copy it out of the most recent entry in CHANGELOG, for example). The benefit of this is that the "releases" tab in github will function as a change log. (For example: https://github.com/vjoel/tupelo/releases.) For this release, you can just paste in these lines from CHANGELOG:

v0.4.2

  * Fix #11, #12, #14.
  * Import lmdb 0.9.12 source.