Fixed deprecation warning from crypto

lib/dynamo/utils/message_verifier.ex:36: crypto:sha_mac/2 is deprecated and will be removed in in a future release; use crypto:hmac/3

Note that this seems not to be backwards compatible for erlang. The guys over at erlang-oauth ended up with an if/else : https://github.com/tim/erlang-oauth/pull/19

Do you want something similar?