Use stable sort for removeProperty of OptionsPropertyGroup.

The original custom sort function is not stable, which causes
different results on HHVM. It now uses the default array_diff
function which is stable. Also updated an incorrect test.

Signed-off-by: Jeff Chan <jefftchan@gmail.com>
Merge pull request #922 from jeffchan/remove-property-stable-sort

Use stable sort for OptionsPropertyGroup removeProperty.