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.
Something doesn't seem right here - on L35, you travel to 15-12-2014. This means that as far as the test environment is concerned, today (i.e. `Date.current`) is that date. Then before all examples you reset the calendar to the present. Then before each example you stub out `Date.current` to return 15-12-2014.

I expected L35 to work its magic all by itself, without the need for the `before(:all)` & `before(:each)` blocks on L43 & L47. And since you call `travel_to` within the ~~`context`~~ `describe` block, `travel_back` happens automatically when the block exits.