change async to false in MandrillTransport

With async set to false, Mandrill is able to return a result similar to this in the its API log:
[
{
"email": "chrispecoraro@gmail.com",
"status": "sent",
"_id": "19fcb428d3324d0f85053d8b2953ccc5",
"reject_reason": null
}
]

or

[
{
"email": "chrispecoraro@gmail.com",
"status": "rejected",
"_id": "19fcb428d3324d0f85053d8b2953ccc5",
"reject_reason": "hard-bounce"
}
]

Otherwise, the result will always be:
[
{
"email": "chrispecoraro@gmail.com",
"status": "queued",
"_id": "19fcb428d3324d0f85053d8b2953ccc5",
"reject_reason": null
}
]

Being able to see the status in the Mandrill API logs helps us to troubleshoot any issues.

From the Mandrill API doc:
async: enable a background sending mode that is optimized for bulk sending. In async mode, messages/send will immediately return a status of "queued" for every recipient. To handle rejections when sending in async mode, set up a webhook for the 'reject' event. Defaults to false for messages with no more than 10 recipients; messages with more than 10 recipients are always sent asynchronously, regardless of the value of async.