Replace postgres_ext with ActiveRecordExtended

[mvgijssel]

https://github.com/GeorgeKaraszi/ActiveRecordExtended

Active Record Extended is the continuation of maintaining and improving the work done by Dan McClain, the original author of postgres_ext.

[Willianvdv]

After working with ActiveRecordExtended and postgres_ext and removing it from h1, I'm not sure if arel_toolkit should support on of these two libraries. Are you open to the idea of removing support for libraries other than Arel itself? It would simplify this gem in complexity and maintainability.

[mvgijssel]

Sure! Would love to remove dependencies 😍