Reduce dependencies

[nichtich]

Directory index should be lightweight. Could you please remove the following dependencies

• php-ref is only needed for debug output - this is unlikely required in production.
• github-api is neither required for normal use. It is only used to check whether a new version of this package exist

Both features could be made optional. In the current form this package cannot be installed using composer with minimum-stability requirements.

[ProjectCleverWeb]

I will look into this, although I can't promise anything at the moment.

This project has collected some dust so I will have to refresh myself on it before making any changes.