Ignore unknown refs instead of throwing an exception

[juanmcasanova]

Solves #160

[GrahamCampbell]

Thanks. Could you add a test covering an example gitlab ref that would have caused an exception?

[juanmcasanova]

Done!

[alexandresalome]

I love it when deleting code solves problems :)

[juanmcasanova]

Travis errors should be fixed now

[phpfui]

Would love to see a new release of this library. I am doing a documentation package that uses the markup files in the docs. The problem is I am validating the html, and there is an error with the md files (someone put in a tag). But I have already submitted a PR (Approved!) to fix this, except it was not stamped as a new version, since it was only a doc change (completely understandable).

Anyway, my unit tests are failing since I want to be a stable version (as going with dev stability introduces other issues).

So long story short, I would really appreciate a new release once this PR merges. Thanks for continuing to maintain this repo, looks like the best one out there!

[GrahamCampbell]

Thanks. :)