Fixes handling of requirements during package update

When a package is updated, the requirements of the current version are deleted first and replaced by the new ones. This way, it's ensured that packages the current version requires and which the updated version doesn't require anymore, are properly handled [cf. WCF 1.1].

Fixes #1286
Merge pull request #1287 from Gravatronics/requirementsUpdate

Fixes handling of requirements during package update