Fixed the message recorder plugin DIC tag

As of v2.2.5, SwiftmailerBundle plugins must be registered using the
swiftmailer.default.plugin tag and not just swiftmailer.plugin anymore.

See https://github.com/symfony/SwiftmailerBundle/blob/v2.2.5/DependencyInjection/Compiler/RegisterPluginsPass.php
Merge pull request #192 from gquemener/bugfix/message-recorder-def

Fixed the message recorder plugin DIC tag