Spcesify the upper version limit of slim-rails
Spcesify the upper version limit of slim-rails

Because the latest version of slim-rails has the following bugs:
* https://github.com/slim-template/slim-rails/pull/135
* https://github.com/slim-template/slim-rails/pull/136

yasaichi/gakubuchi,yasaichi/gakubuchi,yasaichi/gakubuchi