Specifying ‘2.1’ will use latest pre-compiled 2.1 rather than having travis build ruby.
Save the return json from mandrill in the DeliveryMethod so you can access
it in your mailer (if needed).
Updates to ‘Reply-To’ header handling:
 * Use the formatted name which includes ‘display_name’
 * The ‘mail’ gem ensures the formatted name is valid, so this is ok.
 * Include all reply_to email addresses, don’t just take ‘first’ (rfc 822 allows this)
Integration tests verify sent message attributes, minor readability improvements