Fixes create_receiver.

* The integrator API uses a different URL than the regular API calls.
* Rearranges the expected order of params.
* Added notify_url as required.
Fixes to use OpenSSL::Digest.

OpenSSL::Digest::Digest has been discouraged to be used since ruby 1.8.7
(https://github.com/ruby/ruby/blob/ruby_1_8_7/ext/openssl/lib/openssl/digest.rb#L51).

And now it's deprecated (https://github.com/ruby/ruby/pull/446).
Merge pull request #3 from gjorquera/create_receiver

Fixes create_receiver.
Merge pull request #4 from gjorquera/digest

Fixes to use OpenSSL::Digest.