Update lib/devise_invitable/model.rb

Could not send invitations from the rails console according to the usage instructions:  

User.invite!( { :email => 'john.doe@example.com' } )

I poked around, and invite_key_valid was returning false, and the problem seemed to be:

self.class.invite_key.all? do |key, regexp|

self.class.invite_key.all? returned a boolean, not a hash, which seemed to be expected.  After making this change, I could send invitations for new users without issue.