Drop minimum required PHP version to 5.4

- Drupal 8 has a minimum PHP of 5.4.8. Symfony 2.7 still depends on
  5.3.something. 5.4 is a reasonable middle-ground to ensure that new libraries
  targeting older versions of PHP do not need to be written.

Author: weierophinney

.travis.yml

@@ -9,6 +9,7 @@ cache:
 matrix:
   fast_finish: true
   include:
+    - php: 5.4
     - php: 5.5
       env:
         - EXECUTE_CS_CHECK=true

composer.json

@@ -20,7 +20,7 @@
     }
   },
   "require": {
-    "php": ">=5.5",
+    "php": ">=5.4",
     "psr/http-message": "~1.0"
   },
   "require-dev": {