Use `dropOverlappingPermissions` to avoid duplicate injections

If both `https://google.com` and `*://*/*` have been granted, I think this module will attempt to inject the scripts twice on Google.com (although I have not seen this happening)

Using the new [dropOverlappingPermissions](https://github.com/fregante/webext-additional-permissions#dropoverlappingpermissionspermissions) function, this module can clean up the additionalPermissions list before trying to inject the scripts.