Remove monkey patch of activerecord

According to https://github.com/Tonkpils/activerecord-postgres-hstore/commit/7284b8b063293428292666c7290592ea922ca598 and version '0.7.0' of the activerecord-postgres-hstore gem, their monkey patch of active record has been removed and support for `to_hstore` . This causes the monkeypatch this gem has for compatibility with `activerecord-postgres-hstore` to break.