Due to the newly added namespaces, Mailboxer::Notification's STI type should be migrated from 'Message' to 'Mailboxer::Message'.

Otherwise, when upgrading from 0.11 to 0.12 - existing message will "disappear".
Merge pull request #277 from lbeder/master

Migrate Mailboxer::Notification's STI type, when upgrading from 0.11 to 0.12