Update mod.cx_disqus.php

EE 2.6 deprecated generate_json() and wants to use the native json_encode() instead.

Replaced $this->EE->javascript->generate_json() with json_encode() on lines 72-74. Also, converted $entry_id to a string for json_encode(). While most probably leave it as the actual entry_id integer, Disqus does accept any string.