Reformat issue references in changelog

Follow the style of Vagrant [1], for the following reasons:

- [GH-42] instead of #42 makes it more clear that we refer to a Github
  issue
- Putting the reference at the end of each changelog entry makes the
  changelog easier to read, since most users will just care for the
  change, and not for the issue that lead to it.

[1]: https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md