Use official php library

Problem/Motivation

README.md suggest using a library different than the one required by composer, the first is the official library based on API documentation.
It is confusing, specially because the referenced library has a composer.json with a syntax error.
This impacts composer installations.

Proposed resolution

Use https://github.com/disqus/disqus-php as suggested on the README.md.

Remaining tasks

Review.

User interface changes

N.A.

API changes

N.A.

Data model changes

N.A.