Add Travis build step

[mattstratton]

Should crawl all the links in the site now and check for any type of errors, either http status or inside the console

[bridgetkromhout]

Won't a lot of old links probably be bad? Do we want to break if some speaker from 2013 deleted their website or something?

[mattstratton]

I think my theory is that a bad link is a bug on the site and needs to be fixed. So even if someone did delete something, that's a maintenance task to fix it.

I think that maybe it doesn't go in Travis; there's probably some type of other way we can have the site be crawled for bad links and then turn them into issues to fix. Actually, I think that Google's search console does that for us.

In fact, it does:

[mattstratton]

Not needed.