Travis: force PHP 5.3 builds on precise

PHP 5.3.x is not supported on default TravisCI builders (Trusty).

Reference:
https://docs.travis-ci.com/user/reference/trusty#PHP-images

PHP 5.3 is removed from the main php list, and setup as an include in
the build matrix. This is required, otherwise the 5.3 builds end up
being executed twice, once with default 'trusty' and one with 'precise.

.travis.yml

@@ -23,7 +23,6 @@ script:
 #
 
 php:
-  - 5.3
   - 5.4
   - 5.5
   - 5.6
@@ -39,6 +38,14 @@ branches:
 
 matrix:
   include:
+    # PHP 5.3 is not supported under default Travis builder (as of July 2017)
+    - php: 5.3
+      env: DB=mysql
+      dist: precise
+    - php: 5.3
+      env: DB=pgsql
+      dist: precise
+
     # Add a specific build for Documentation
     - env: DOCBOOK=1
       # Using Ruby as it's preinstalled and we don't need PHP to build docs