Fix an error to display pgp message with Enigmail 1.5.1

I've fixed one more bug with Enigmail 1.5.1. It's caused by Enigmail code change.

enigmailSvc.mimeInitialized() was removed.
http://sourceforge.net/p/enigmail/source/ci/0a50a6d763b46761e283fc01ba1dd766ab2ddd88