Digest::Digest is deprecated

Use of OpenSSL::Digest::Digest has been discouraged since Ruby 1.8.7 and generates a deprecation warning on Ruby 2.1.x

See: https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51 and https://github.com/ruby/ruby/pull/446

This would necessitate a bigger version bump in the next release, as this would make the gem backwards-incompatible with clients that pass `Digest::Digest` instead of just `Digest`.