Increase minimum required PHP version to 5.5

See mailinglist discussion at https://groups.google.com/forum/#!topic/silverstripe-dev/7mVvhNNeNps

.travis.yml

@@ -20,9 +20,9 @@ env:
 
 matrix:
   include:
-    - php: 5.4
+    - php: 5.5
       env: DB=MYSQL
-    - php: 5.4
+    - php: 5.5
       env: DB=PGSQL
     - php: 5.5
       env: DB=SQLITE

composer.json

@@ -16,7 +16,7 @@
 		}
 	],
 	"require": {
-		"php": ">=5.3.3",
+		"php": ">=5.5.0",
 		"composer/installers": "*",
 		"silverstripe/framework": "4.0.x-dev",
 		"silverstripe/reports": "*",