Enhancement: Explicitly configure build matrix to maintain order

.travis.yml

@@ -16,22 +16,20 @@ addons:
     packages:
       - parallel
 
-php:
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - nightly
-
 matrix:
   include:
     - php: 5.3
       dist: precise
+    - php: 5.4
+    - php: 5.5
+    - php: 5.6
+    - php: 7.0
+    - php: 7.1
+    - php: 7.2
+    - php: 7.3
     - php: 7.3
       env: deps=high
+    - php: nightly
   fast_finish: true
   allow_failures:
     - php: nightly