Replace CHANGELOG with release notes in annotated tags

From https://github.com/tpope/git-bump/issues/7#issuecomment-46694410:

    "If you save a GitHub Release for an annotated tag without setting the
    title or body, they will be pre-populated from the tag's message."

So we change the release rake task to populate the message body of the
annotated tag that we create instead of opening the CHANGELOG. These
annotated tag message bodies can later be used to prepopulate the
release notes when creating a new release from a tag on the Github web
interface.

Release notes after 2.4.0 (when Chocolate was open sourced) have been
manually copied over to the GitHub releases page.