capture result from mandrill, use formatted name for reply-to header

Hi, I appreciate your recent cleanup of mandrill_dm in this fork!  Not sure what your intentions are long-term, but it looks like the upstream mandrill_dm is dead.  Please let me know if you aren't interested in this stuff.

This PR does a couple things:
 * let travis use latest pre-compiled ruby 2.1 rather than forcing compile
 * capture mandrill return value so you can access it via `<mail object>.delivery_method.result`
 * use the formatted email address for reply-to headers and allow multiple reply-to addresses.