TravisCI: sort PHP versions in descending order

This way, more recent versions are built first.

Issue #27829

.travis.yml

@@ -32,11 +32,11 @@ script:
 #
 
 php:
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
   - 8.0
+  - 7.4
+  - 7.3
+  - 7.2
+  - 7.1
 
 env:
   - DB=mysql
@@ -45,16 +45,16 @@ env:
 jobs:
   include:
     # Use 'xenial' build environment for legacy PHP versions
-    - php: 5.6
+    - php: 7.0
       dist: xenial
       env: DB=mysql
-    - php: 5.6
+    - php: 7.0
       dist: xenial
       env: DB=pgsql
-    - php: 7.0
+    - php: 5.6
       dist: xenial
       env: DB=mysql
-    - php: 7.0
+    - php: 5.6
       dist: xenial
       env: DB=pgsql