[zfcampus/zf-apigility#28] Raise exception if unable to serialize JSON

- Updated the JsonModel to raise an exception if unable to serialize the data as
  JSON.
[#14] CS fixes

- trailing whitespace
[#14] Mark test as skipped for PHP < 5.5.0

- Evidently, PHP versions less than 5.5 are more lenient in character
  encoding detection. The logic is still valid; just harder to get a
  reasonable test case.
Merge branch 'hotfix/zf-apigility-28'

Fixes zfcampus/zf-apigility#28