Fix occasionally failing start_date specs

Pivotal Bug https://www.pivotaltracker.com/story/show/77245130.

Converted all uses of `Date.today` to `Date.current`. See [here](http://nikitaavvakumov.github.io/posts/date-in-ruby-vs-rails/) for explanation.
I agree, the validator knows too much about the model, ideally you could rename attributes to `start_date`