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.
This isn't a big deal by any stretch, but as a general rule when I see a static date it starts ringing an alarm bell that the test may be brittle.
There is nothing wrong with what you have done but it potentially gives an impression that is unwarranted because the code and tests are good.
I wouldn't lose sleep if what you have above was committed but have a look at the tests for the sponsor spec, see what has been done there, and what you think of it.