Placeholder\Registry::unsetRegistry() should only be used with < 2.2.0 

Currently the ZF2 module will trigger errors, causing failures with ZF >= 2.2.0, as the placeholder view helpers no longer use singleton registries.
Added a compareVersion call to check for < 2.2.0 to still conditionally reset the singleton registry for only appropriate versions of ZF2.
Merge pull request #669 from Bittarman/patch-1

Placeholder\Registry::unsetRegistry() should only be used with < 2.2.0