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 hi! Just a friendly reminder, that it would be great to understand if multi_json is given or need to be explicitly required