Make travis not fail deploys on external link checker

[azaroth42]

e.g. from https://travis-ci.org/IIIF/api/builds/308051148

- ./_site/api/image/2.0/index.html
  *  External link https://profiles.stanford.edu/stuart-snydman failed: 500 No error
rake aborted!
HTML-Proofer found 1 failure!

Shouldn't prevent us from deploying.

Or a master PR: https://travis-ci.org/IIIF/api/builds/308053764

[glenrobson]

The link above was a transient error as the URL does still exist at that location. To fix I clicked 'restart build' in travis and the second time through it finished correctly.

On the wider question on whether you should validate external links I would think it a benefit to ensure all external links work before publishing to the live website. I appreciate this could be relaxed on branches but would think it should be left in place on master.

[glenrobson]

Regarding the errors on master:

- ./_site/api/annex/notes/jsonld/index.html

  *  External link http://json-ld.org/spec/latest/json-ld-framing failed: 404 No error

- ./_site/api/image/1.1/change-log/index.html

  *  External link http://json-ld.org failed: 404 No error

- ./_site/api/image/2.1/index.html

  *  External link http://www.json-ld.org failed: 404 No error

- ./_site/test/api/annex/notes/design_patterns/index.html

  *  External link http://json-ld.org/spec/latest/json-ld-api-best-practices failed: 404 No error

It looks like the json-ld.org site has moved over to https so I would suggest a build warning notifying you of the change might be useful. There does seem to be a bug in the HTMLProofer code that it doesn't see a 301 but complains of a 404. 301s are already ignored so won't stop the build process.

[glenrobson]

Just rebuilt the master branch and it looks to have passed now so unless you've fixed the source the 404s could have been transient errors too.

[jpstroop]

We used to only have the site check the links every week or two, and htmlproofer was supposed to be caching the results in this file: https://github.com/IIIF/api/blob/master/tmp/.htmlproofer/cache.log

The frequency is set here: https://github.com/IIIF/api/blob/master/Rakefile#L29

Now realizing (or maybe remembering) that that doesn't work since Travis can't write to the repo. If we do want to have the links checked periodically but not have it block progress on other PRs, maybe we need a cron to run the check somewhere else? Or we just need top remember to do it once and a while.