Create HSTORE field for Customer identifier name/value pairs on trip tickets

* Changing AR schema format setting in preparation for using hstore
* Updating Gemfile to use versioned activerecord-postgres-hstore instead of edge from GitHub
* Adding hstore setup migration
* Adding indexed trip_tickets.customer_identifiers hstore field to schema
* Rudimentary implimentation of hstore fields in form view
* Making hstore key fields dynamic
* Added ability to remove hstore attribute pairs
* Adding method to add hstore attribute pair fields
* Removing all attribute field pairs will set the customer_identifiers field to blank
* Fixing error on trip ticket form view
* Setting schema_format config option back to the default since using SQL was causing problems when running db:test:prepare
* Adding integration tests for customer identifier attribute fields
* Fixing javascript issue when removing all customer identifier attribute pairs