Stop using deprecated OpenSSL::Digest::Digest

OpenSSL::Digest::Digest is deprecated as of Ruby 2.1 and has been
discouraged since Ruby 1.8.7.

See: http://goo.gl/E4O0Oq
Don't use intermediate OpenSSL::Digest

cc @tmm1
Merge pull request #103 from chrishunt/drop-a-digest

Stop using deprecated OpenSSL::Digest::Digest