Fix pool size

- pool in db.yml -> 25 for sidekiq
- set db pull for unicorn to 2

Info
- https://devcenter.heroku.com/articles/concurrency-and-database-connections#connection-pool
- http://stackoverflow.com/questions/18899464/possible-to-avoid-activerecordconnectiontimeouterror-on-heroku
- http://stackoverflow.com/questions/8663529/really-need-a-db-connection-pool-for-unicorn-rails