Travis CI should verify that all links in the documentation are valid.

[billonahill]

See website/README.md for how we do this manually. We should do this in Travis CI to make sure we don't commit bad links.

[objmagic]

a small example can be found here

[objmagic]

close this since #690 is merged?

[billonahill]

#690 was auto-closed by the git history cleanup and never submitted. #709 actually included the fix, but the check is currently disabled until #346 is done and the bad links are fixed.

[objmagic]

bad links should be gone after #734, #738, #739