Build/Test Tools: Enable persistent object cache on a PHP 7.3 build instead of PHP 5.6.

Props netweb, johnbillion.
Fixes #46633.

git-svn-id: https://develop.svn.wordpress.org/trunk@45027 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -18,14 +18,14 @@ matrix:
     env: WP_TRAVISCI=travis:js
   - php: 7.4snapshot
   - php: 7.3
+  - php: 7.3
+    env: WP_TRAVIS_OBJECT_CACHE=true
+    services: memcached
   - php: 7.2
   - php: 7.1
   - php: 7.0
     env: WP_TEST_REPORTER=true
   - php: 5.6
-  - php: 5.6
-    env: WP_TRAVIS_OBJECT_CACHE=true
-    services: memcached
   - php: 5.5
   - php: 5.4
   - php: 5.3