Build/Test tools: Further trimming of CI jobs on the 4.9 branch.

This removes the PHP 5.6 job which runs without an object cache in place as the likelihood of a change being backported that only breaks 5.6 environments without an object cache is small.

Merges [45005] into the 4.9 branch.

See #42387


git-svn-id: https://develop.svn.wordpress.org/branches/4.9@45006 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -15,7 +15,6 @@ matrix:
   - php: 7.1
     env: WP_TRAVISCI=travis:js
   - php: 7.2
-  - php: 5.6
   - php: 5.6
     env: WP_TRAVIS_OBJECT_CACHE=true
     services: memcached