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

OpenSSL::Digest::Digest has been discouraged to use since Ruby 1.8 https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
and was deprecated recently. ruby/ruby#446

This removes the deprecation warning shown in ruby 2.1 and above.