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).
@kalabiyau: Hello, sorry for the late response.  As of sensu/sensu@3f442466278b43825c4dc2a512169bc78b1bf94d, it looks like Sensu (and, consequently, the omnibus package) now uses MultiJson in place of Oj.
If you think we'd be better served by keeping Oj and adding it as a requirement for the plugin, I'll happily close the PR. Thanks!