Update some MELPA references
[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.
Merge pull request #891 from pw4ever/issue867

[Fix #867] Update Grimoire URL to fix (cider-grimoire-lookup) regression due to HTTP 301 (Moved Permanently)
Mention Marmalade in the installation instructions