kiji-modeling: EXP-274. Fix modeling for new Avro API introduced by EXP-222

review: https://review.kiji.org/r/1325
issue: https://jira.kiji.org/EXP-274
commit: https://github.com/kijiproject/kiji-modeling/commit/23c682f1c2ed44cfbf4de6dcf0fd96960d503b0d