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'm less happy about the `tomorrow` solution as it requires a bit of reasoning before understanding that the condition will never be a failing one.
Keeping the first date static ensures reproducibility, and that is valuable _if_ we are certain that all relevant conditions for the test are covered. Otherwise, randomly dated tests might uncover errors that occur on particular days.
I believe that we are covering all bases here, so would opt for a static date.