update Listen message send to use new Listen 2.2.0 API

This pull request updates Mailman to use the latest version of Listen which today is '2.2.0'.  Listen 2.2.0 has a new API that breaks Mailman. Mailman's gemspec specifies a dependency on Listen with '~> 1.3.1' which to my knowledge means >= 1.3.1 and < 2.0, however Listen 2.2.0 was installed for me with Mailman so I chose to deal with it.