Modify CHANGELOG to use header elements

Add a title and make each of the version numbers an `h2` so the
CHANGELOG is a little easier to read.

Also made a small change to a linter name so syntax highlighting for the
bottom portion of the document looks right (the underscore was
triggering Markdown's "emphasis" syntax highlighting).

Change-Id: I421ed95bdc34d97cbf33714d590666764af08b13
Reviewed-on: https://gerrit.causes.com/28948
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: David Gouldin <david.gouldin@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>