Switching ArrayObject matcher to try and fix build for HHVM

TBH I just opened this as an experiment because I don't have local HHVM, I had a thought that it might work.

If it passes I'll refactor to skip isset only on HHVM. 

This is failing against the 'hhvm' travis target, and it is a reported bug in HHVM but we need to work around it in the meantime if we're going to support the platform.