Fix Digest::Digest warning

This closes the problem @preillyme (and everyone else) gets on > 2.1. It's caused by an old @coinbase gem which used Digest::Digest. FYI  ``OpenSSL::Digest::Digest has been discouraged to use from very ancient era such as Ruby 1.8 and finally was deprecated recently.``, see: http://stackoverflow.com/questions/21184960/ruby-digestdigest-is-deprecated-use-digest.

This depends on #14 as I don't like submodules :+1: