Improves performance of the live departures query

1) Improves performance of the stations_update_json method in live_controller:
* count of live messages for each basic_schedule iterated doesn't issue a separate db query now
* finding the first live message also doesn't issue a separate db query now

2) Refactors the runs_between method in the location model so it uses scopes more efficiently:
* reduces number of db queries
* it also limits the select arguments in the runs_between method in location.rb to only what's needed

3) Improves the json generation time via the oj gem
Merge branch 'feature-faster-live-departures-query' into development