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.
When I was wrote this validator, I created it only for the `Partner` model and had planned to convert it to a `EachValidator`. But I could never come back it to it. When thats done, it'll look much better as you will be able to pass in the attribute to validate.