Speed up retrying of failed external link checks

[aroben]

We retry external link checks in a few situations (e.g., to see if the server supports GET but not HEAD). But we were performing these retries in a blocking manner. We now put the retries into the Typhoeus::Hydra queue just like all the other requests, so they can be performed in parallel. This speeds up checking 62 links on a server that does not support HEAD from 20 seconds to 4 seconds on my machine.

[gjtorikian]

I was about to question this, but it's true! I can't believe our Help site doesn't accept HEAD requests:

$ curl -i -X HEAD https://help.github.com/articles/fork-a-repo
HTTP/1.1 404 Not Found
Cache-Control: no-cache
Content-Type: text/html; charset=utf-8
Date: Fri, 08 Aug 2014 17:00:15 GMT
Set-Cookie: _help_session=....-0000; secure; HttpOnly
Status: 404 Not Found
Strict-Transport-Security: max-age=31536000
X-Content-Type-Options: nosniff
X-Frame-Options: deny
X-Request-Id: 733c986c-c4f6-4e3d-887d-ee798483bfe5
X-Runtime: 0.009595
X-Xss-Protection: 1; mode=block
Connection: keep-alive