O295and o296 sponsorship start date validation

https://osraav.atlassian.net/browse/OSRA-295
https://osraav.atlassian.net/browse/OSRA-296

Model validation for Sponsorship.start_date 
- date can't be blank
- date can't be an arbitrary string
- date can't be later than the first of next month

plus rspec  and cucumber tests

Needs ruby 2.1.5
Me and Andrey paired on all this.
after taking out

after (:all) do
  travel_back
end

I tested if the date is set back by default after exiting the describe block and it looks like it is.
I added a puts Date.current in the next describe block and it was reset back to today.
So travel_back is not needed