ref(Makefile): use gen-changelog.sh from go-dev

[mboersma]

[skip ci]

Summary of Changes

Replaces the local _scripts/generate-changelog.sh with the improved new standard version gen-changelog.sh in the quay.io/deis/go-dev:0.12.0 image.

If this approach is acceptable, a slew of copycat PRs are to follow. So I would appreciate feedback on whether make update-changelog is a good target name, if this could be further simplified, and whether it automates enough of the Deis Worfklow release process before I forge ahead.

cc: @bacongobbler @vdice

Issue(s) that this PR References

deis/docker-go-dev#35.

Testing Instructions

1. Run make update-changelog
2. Run git diff and examine the output
3. Open CHANGELOG.md in an editor

Expected Results:

1. CHANGELOG.md should have a new section at the top
2. The section header should reference the latest tag in your local builder codebase
3. The section header should reference HEAD (or $DEIS_RELEASE if that env var is set)
4. The updated CHANGELOG.md should render correctly as Markdown
5. Links in the updated CHANGELOG.md should work

   Pull Request Hygiene TODOs

Please make sure the below checklist is complete.

• [X] Your pull request is concise and to the point (make another PR for refactoring nearby code)
• [X] Your commits are squashed into logical units of work
• [X] Your commits follow the commit style guidelines