Stop maintaining the changelog

All releases information has been migrated to GitHub Releases:
https://github.com/sstephenson/ruby-build/releases

This frees core contributors from having to maintain release notes in version
control, and regular contributors from wondering whether they need to edit
CHANGELOG while submitting a pull request.

Benefits of release notes being outside of version control are that they can be
edited post-release.
Merge pull request #545 from sstephenson/nuke-changelog

Stop maintaining the changelog