Sequel now correctly interprets arrays as postgres arrays

Before, the pg_arrays Sequel extension wasn't being loaded correctly to utilize arrays in the model. 

This fixes `Sequel::DatabaseError - PG::Error: ERROR:  missing dimension value` showing up when receiving the `tags` array from Orbiter, etc.

Fixes helios-framework/helios#21

[source](http://www.ruby-forum.com/topic/3984710)