Updated flapjack.rb for 0.13

Currently, the flapjack handler isn't working with the new 0.13 release.

This change updates references from `Oj` to `MultiJson`.

Tested with Sensu 0.13 (omnibus) and Flapjack 0.9.3 (omnibus).
@clarkf thank you for response. I think MultiJson is absolute fine as it is actually just a wrapper above json engines. If user has a oj, then it will be used. Thanks!