Support activerecord4.0 over

Fixed Issue alfa-jpn/kakurenbo#8 .
Support Rails4.0 and 4.1 over.

#### Changed hard-delete api interface.

From `model.destroy!` To `model.destroy(hard: true)`

##### As a reason,
- Rails4.0 over, `destroy!` method is used as activerecord original method.
[ActiveRecord::Persistence#destroy!](http://apidock.com/rails/v4.0.2/ActiveRecord/Persistence/destroy%21)

- when destroy association  using `dependent :destroy`, `destroy!` method is used.
https://github.com/rails/rails/blob/4-1-stable/activerecord/lib/active_record/associations/has_many_association.rb#L109-L113