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.
The `before(:all)`/`after(:all)` implementation works for me. 
Not sure what you gain with a hash instead of individual `let`s, and I'm personally not a fan of all the blank space on L49-58, but these are minor things.