[Fix #867] Fix (cider-grimoire-lookup) regression due to HTTP 301 (Moved Permanently) to the old Grimoire URL.

Grimoire recently migrates to "http://conj.io". The old URL used in
(cider-grimoire-lookup) would drop the "Content-Type: text/plain"
request enroute of a HTTP 301 redirection.

Updating the Grimoire URL to "http://conj.io" fixes the issue.