[Gemspec] Upgrade xcodeproj dependency to match new cocoapods verison's.
[Gemspec] Upgrade xcodeproj dependency to match new cocoapods verison's.

mattdelves/slather,viteinfinite/slather,RyanCopley/slather,nerdishbynature/slather,nerdishbynature/slather,petester42/slather,fikeminkel/slather,neonichu/slather,robertofrontado/slather,fikeminkel/slather,tabuchid/slather,domenicosolazzo/slather,domenicosolazzo/slather,tabuchid/slather,nerdishbynature/slather,nerdishbynature/slather,SlatherOrg/slather,domenicosolazzo/slather,mattdelves/slather,orta/slather,RyanCopley/slather,orta/slather,petester42/slather,AliSoftware/slather,tabuchid/slather,tabuchid/slather,petester42/slather,petester42/slather,venmo/slather,dhardiman/slather,RyanCopley/slather,ben-ng/slather,mattdelves/slather,mattdelves/slather,xing/slather,dmiedema/slather,orta/slather,tarbrain/slather,xing/slather,xing/slather,fikeminkel/slather,fikeminkel/slather