Fix an error to display pgp message with Enigmail 1.5.1

enigmailSvc.mimeInitialized() was removed.
http://sourceforge.net/p/enigmail/source/ci/0a50a6d763b46761e283fc01ba1dd766ab2ddd88
Merge pull request #723 from shirosaki/enigmail

Fix an error to display pgp message with Enigmail 1.5.1