Switch all usages of Date.today to Time.zone.today

So I happened to be taking a look at this right as midnight rolled around and all my tests started failing again, seems there's some timezone issues with using `Date.today` along with the `ActiveSupport::TimeWithZone` methods like [`#today?`](http://api.rubyonrails.org/classes/ActiveSupport/TimeWithZone.html#method-i-today-3F).

An alternative that would likely work as well for the purposes of the meetup is to change `config/application.rb:36` to NZST. The tests would then likely work whether implementations use `Date.today` or the `Time.zone` methods. Doing it this way may force students to learn proper time zone handling, naïve implementations of `days_until_due` will be half a day out.