PHP 5.6 test failures shouldn't be allowed.

PHP 5.6 was released on 28 August 2014 and thus tests on it should be on the same level as all of our other PHP tests pass.

Fixes #29682
Props netweb



git-svn-id: https://develop.svn.wordpress.org/trunk@29844 602fd350-edb4-49c9-b593-d223f7449a82