Build/Test Tools: Remove PHP 7.1 from allowed failures

PHP 7.1 has been officially released. http://php.net/archive/2016.php#id2016-12-01-3

See #37625
Props helen, pento, The PHP team

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git-svn-id: https://core.svn.wordpress.org/trunk@39364 1a063a9b-81f0-0310-95a4-ce76da25c4cd