Don't quote the default issue closing regex.

It turns out that regex escapes and YAML quoted-scalar escapes don't play
nicely together.
Merge pull request #5093 from smashwilson/example-config-fix

Fix issue_closing_regex in gitlab.yml.example
Update gitlab_git to version 2.2.0. Fixes invalid diff in Compare
Update docs to use 6-1-stable branch