Fix wrong arguments given to Net::IMAP constructor, it should take 2 arguments host and options.

ssl option can be hash now, in this case Net::IMAP pass it to OpenSSL::SSL::SSLContext#set_params, so we can pass options like:

Mailman.config.imap = {
  ssl: { verify_mode: OpenSSL::SSL::VERIFY_NONE },
  ...
}