Use Release Drafter's draft release when publishing a new release with the Rakefile

[interstateone]

Currently the Rakefile will make a new release that points at the newly created tag. It would be nice to update the draft release, if one exists, to point at that new tag and then publish it. Documentation should be updated to include a step to polish the release copy before using the Rakefile.

https://github.com/toolmantim/release-drafter/