Use lambdas for scopes.

https://trello.com/c/xOGwegeZ

The scopes in the event models were not in lambdas, which was causing
issues when the date was being evaluated. The model has been updated so
that the date is evaluated every time a scope on an event model is
called.
Set up Unicorn for Heroku.

https://trello.com/c/6uzQE6Va

Previously, we were experiencing some issues when Unicorn was starting
up on Heroku, which was causing Postgres errors to be raised. The
Unicorn processes have been configured inline with Heroku's
documentation.