Use OpenSSL::Digest instead of deprecated OpenSSL::Digest::Digest

OpenSSL::Digest::Digest has been deprecated because it is from an older Ruby 1.8 era https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
and finally is giving deprecation warnings with Ruby 2.1.0: ruby/ruby#446