Detect and link-ify github commit ref in log

Addresses https://github.com/travis-ci/travis-ci/issues/1470
Rearrange github link helpers

So that regex and transformation appear together.
HTTPS everywhere for github.com
Add specs for link-ifying commit references
Merge pull request #246 from travis-ci/ha-bug-gh_ci_1470

Link-ify GitHub commit references in logs