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

OpenSSL::Digest::Digest has been discouraged to use from very ancient era such as Ruby 1.8 https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
and finally was deprecated recently. https://github.com/ruby/ruby/pull/446