Sponsor start date validation before 1st of next month

 https://osraav.atlassian.net/browse/OSRA-116

pair programming implementation of story
OSRA-116 #Code-Review
:smile: Kudos for building it up from basic principles, but... http://apidock.com/rails/DateAndTime/Calculations

```ruby
Date.current.beginning_of_month.next_month
```

As a side note,
```ruby
Date.parse('January 30, 2014') + 1.month # => Fri, 28 Feb 2014
Date.parse('January 30, 2014').next_month # => Fri, 28 Feb 2014
Date.parse('January 30, 2014') + 30.days # => Sat, 01 Mar 2014
```
Something to be aware of.