Refactor incoming mail hook for Mailgun support

[svoop]

In order to implement other hooks e.g. for Mandrill:

• Add a private method "TicketsController#mandrill" which returns the raw incoming message (compatible to the output of the post-mail script).
• Add "mandrill" to the list of constraints of the "post /:hook/:key/tickets.json" route.

For more, see #436

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:warning: WARNING :warning:

The URL for the mail hooks had to be changed in order to accomodate other mail providers. This commit thus breaks things for those already using the post-mail script. However, the change needed is no biggie at all and mentioned in the CHANGELOG:

First find out the mail key which protects the mail hook:

rake secret:mail_key

Then update the aliases file of your MTA from the old to the new hook URL (replace {MAIL_KEY} with the output from above):

OLD: http://yoururl.com/tickets.json
NEW: http://yoururl.com/post_mail/{MAIL_KEY}/tickets.json

[coveralls]

Coverage decreased (-1.5%) to 90.567% when pulling 1e0f612ba7f57b0cc3649456f0875b58874ac2f6 on svoop:mailgun into 877ed79f1465fbd3c697f0e060ed0f43e9b36dfa on ivaldi:master.

[coveralls]

Coverage decreased (-1.5%) to 90.567% when pulling 1e0f612ba7f57b0cc3649456f0875b58874ac2f6 on svoop:mailgun into 877ed79f1465fbd3c697f0e060ed0f43e9b36dfa on ivaldi:master.

[coveralls]

Coverage decreased (-1.5%) to 90.567% when pulling 1e0f612ba7f57b0cc3649456f0875b58874ac2f6 on svoop:mailgun into 877ed79f1465fbd3c697f0e060ed0f43e9b36dfa on ivaldi:master.

[coveralls]

Coverage decreased (-2.1%) to 90.005% when pulling c2b300606ec5cb95e113c2d49605f089d5e8e733 on svoop:mailgun into 877ed79f1465fbd3c697f0e060ed0f43e9b36dfa on ivaldi:master.

[svoop]

Thanks a lot, @frenkel