Have log_out method return json

This commit helps improve the discourse_api experience so that we can
check the json response if it was a success or not. Please see the
discourse_api pull-request #50 for more info:

https://github.com/discourse/discourse_api/pull/50

I'll hopefully have time this weekend to submit another pull request to address
the 500 error for an invalid user_id in log_out.