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.

/cc @sstephenson @sferik #158 #251