`Whip_RequirementsChecker::addRequirement()` loses requirements

[schlessera]

The Whip_RequirementsChecker::addRequirement() method only adds requirements for a component if that component has not yet been added.

This should first compare the version requirements when dealing with duplicate components, and only keep the most strict version requirement.

Otherwise, if you add php >= 5.2 and then php >= 5.6, the 5.6 requirement is skipped because the component php was already added.