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.
we do on line 12:
validate  :start_date_no_later_than_1st_of_next_month, if: :start_date

and on line 13 we validate for presence of start_date:
validates :start_date, presence: { scope: true, message: "is invalid"}