Use version_compare() for extension version comparison

CiviCRM's extensions page rolls its own version comparison via explode/foreach.

version_compare() is a better way to do this, because it correctly identifies the following version comparisons which IMO the existing implementation gets wrong -

2.1 >= 1.1
1.0.1 >= 1.0
1.1-beta1 <= 1.1

It's not clear why CRM_Core_Extensions::getExtensions() has its own logic - version_compare() is used in ~150 places throughout CiviCRM.