minor #5148 Upgrade Xcode (kubawerlos)
minor #5148 Upgrade Xcode (kubawerlos)

This PR was merged into the 2.15 branch.

Discussion
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Upgrade Xcode

Builds are currently failing e.g. https://app.circleci.com/jobs/github/FriendsOfPHP/PHP-CS-Fixer/9285.

So let's try the simplest solution (FYI [Supported Xcode Versions](https://circleci.com/docs/2.0/testing-ios/#supported-xcode-versions)).

Commits
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639a4924c Upgrade Xcode

GrahamCampbell/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer,GrahamCampbell/PHP-CS-Fixer,keradus/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,keradus/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,FriendsOfPHP/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,Slamdunk/PHP-CS-Fixer,gharlan/PHP-CS-Fixer,SpacePossum/PHP-CS-Fixer,kubawerlos/PHP-CS-Fixer