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.
I looked at the sponsor specs and day in future but not past next month is not tested ( except first_of_next_month). I also want to test five_days_ahead_this_month or at least tomorrow.

I also checked with different dates and it looks stable.
Let's discuss more at the scrum.