* Add support for PHPUnit 6+.
* Add Composer files to the cache on Travis.
* Remove HHVM from the test infrastructure on Travis.
Merges [40536], [40538], [40539], and [40546] to the 4.7 branch.
See #40539Fixes#39822, #40548
git-svn-id: https://develop.svn.wordpress.org/branches/4.7@40547 602fd350-edb4-49c9-b593-d223f7449a82