Fix the configuration of the PHP 5.3 job on precise

Env variables defined as matrix are not merged with the job env variables
of the matrix. We need to use global env variables (which we can still
override in a specific job if needed).

Author: stof

.travis.yml

@@ -9,7 +9,8 @@ cache:
 php: [5.4, 5.5, 5.6, 7.0, 7.1, 7.2]
 
 env:
-  - WEBDRIVER=selenium
+  global:
+    - WEBDRIVER=selenium
 
 matrix:
   fast_finish: true