fix issue #111 (adapt models/check.rb to latest sensu api version 0.12)

Hi!

I think issue #111 is faced with this problem.
I update models/check.rb to fix this problem.

* latest

http://sensuapp.org/docs/0.12/api-checks

>/request
>success: 202

* old version

http://sensuapp.org/docs/0.11/api-checks

> /check/request
> success: 202

Hiroaki