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

In GroupDocs::Api::Helper::URL in sing_url method use OpenSSL::Digest
instead of deprecated OpenSSL::Digest::Digest.

Refer to:
https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
https://github.com/ruby/ruby/pull/446