Fix broken link to ActiveJob documentation

Since ActiveJob is in a beta release, the current link returns 404:

http://api.rubyonrails.org/classes/ActiveJob/QueueAdapters.html

In order to have a valid link, the URL must be:

http://edgeapi.rubyonrails.org/classes/ActiveJob/QueueAdapters.html

I think it's better to point at edgeapi than to a broken link.
The original URL can be restored as soon as 4.2.0 is officially released.

[ci skip]