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.
> travel_to isn't receiving a block

It doesn't need to — see examples in Rails API: http://api.rubyonrails.org/classes/ActiveSupport/Testing/TimeHelpers.html#method-i-travel_to