Build/Test Tools: Trim the test matrix on Travis in order to speed up the 5.1 branch build.

This removes the PHP 7.2, 7.1, 5.5, 5.4, and 5.3 jobs.

This also 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 [44992] and [45005] to the 5.1 branch.
See #42387, #40407.

git-svn-id: https://develop.svn.wordpress.org/branches/5.1@47322 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -17,18 +17,11 @@ matrix:
   - php: 7.1
     env: WP_TRAVISCI=travis:js
   - php: 7.3
-  - 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
-    dist: precise
   - php: 5.2
     dist: precise
 before_install: