Also build against PHP 5.5.

But a failing build agains PHP 5.5 should not fail the whole build.

.travis.yml

@@ -2,6 +2,11 @@ language: php
 php:
   - 5.3
   - 5.4
+  - 5.5
+
+matrix:
+  allow_failures:
+      - php: 5.5
 
 before_script:
  - composer selfupdate --quiet