Fix the bug of "if" in validates_presence_of :collapse_key

`:time_to_live?` is always evaluated as `true`, so `validates_presence_of :collapse_key` is always enabled in spite of whether `time_to_live` is `nil` or not.

You might fix it from `if` to `, if:`.

References;

* http://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_presence_of
* http://stackoverflow.com/questions/10501401/validates-presence-of-if-condition-on-rails-3-2-and-mongoid-simple-form