Update Message Builder doc

[embold-tyler]

Message Builder.md references deprecated methods. I utilized the method comments of the MessageBuilder class to build a command, but using the doc on its own resulted in lots of deprecation notices. I didn't find the reply_to method in any of the docs either. The Mailgun API documentation mentioned setting it with h:reply-to, but the the comment implies that method is also deprecated in favor of MessageBuilder.reply_to as well. The Ruby SDK Reference linked from README.md is sparse and doesn't even mention the MessageBuilder class. MessageBuilder.md links to the API doc for further examples but those are not specific to the Ruby SDK.