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).
is that is not necessary to add MultiJson as a requirement for this plugin?