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.
`context` is preferred over `describe` when it works better semantically. e.g. `context 'when user is signed in' do` - i.e. "run some tests in the context of a user being signed in". In this case, `describe 'start_date' do` is more appropriate - i.e. "describe valid and invalid values for `start_date`".