Update admin booths controller

[rohitverma007]

In app/controllers/admin/poll/booths_controller.rb, update available method to use available_for_admin method on Poll::Booth instead of available method.
Also, in spec/models/poll/booth_spec.rb, add some tests to to test the available_for_admin method on Booth model. I will provide poll and booth models for reference. Do not update booth.rb and poll.rb model files.

https://github.com/rohitverma007/consuldemocracy/blob/master/app/controllers/admin/poll/booths_controller.rb

https://github.com/rohitverma007/consuldemocracy/blob/master/spec/models/poll/booth_spec.rb

https://github.com/rohitverma007/consuldemocracy/blob/master/app/models/poll.rb

https://github.com/consuldemocracy/consuldemocracy/blob/master/app/models/poll/booth.rb

[rohitverma007]

closed by #5 , good job codie