Specify dist for 5.3

.travis.yml

@@ -1,24 +1,22 @@
-
 language: php
 
 sudo: false
 
 dist: trusty
 
-php:
-#  - 5.2 # we cannot run 5.2 because phpunit is installed using composer
-  - 5.3
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
-  - nightly
-
 matrix:
+  include:
+   - php: 5.3
+     dist: precise # PHP 5.3 is supported only on Precise.
+   - php: 5.4
+   - php: 5.5
+   - php: 5.6
+   - php: 7.0
+   - php: 7.1
+   - php: 7.2
+   - php: 7.3
+   - php: 7.4
+   - php: nightly
   fast_finish: true
   allow_failures:
    - php: nightly