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

Even though there has been no deprecation warning until now OpenSSL::Digest::Digest has only been
provided for backwards combability since Ruby 1.8

See https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51
Deprecation warning added with https://github.com/ruby/ruby/pull/446