Add "broken link checker" to continuous integration tests

[gtback]

I've got a script lying around I've used to do this on an ad-hoc basis. I'll have to clean it up and make it available on Travis.

[guitarmanvt]

Broken link on http://stixproject.github.io/documentation/concepts/creating-quality-indicators/ to http://stixproject.github.io/language/profiles/samples/stix_1.2_sample_indicator_sharing_profile_r1.xlsx

[gtback]

Yes, the profile XLSX is still on stix.mitre.org not stixproject.github.io, as far as I can tell.

[gtback]

In terms of actual link checkers, I would recommend using this one. My pretty-basic attempt is here, and I've also had success with this one.