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.
My fault, I'd misunderstood the docs. @azangru, @razorcd can you please add the `travel_back` step or rewrite as
```ruby
travel_to Date.parse "15-12-2014" do
  ..tests..
end
```