Add more platforms to Travis build matrix

This ensures we test against old and current Symfony versions

.travis.yml

@@ -1,7 +1,27 @@
 language: php
 
+sudo: false
+
 php:
   - 5.3
   - 5.4
+  - 5.5
+  - 5.6
+  - 7.0
+  - hhvm
 
-before_script: composer install --dev
+matrix:
+    fast_finish: true
+    allow_failures:
+        - php: hhvm
+        - env: SYMFONY_VERSION=3.0.*
+
+    include:
+        - php: 5.3
+          env: COMPOSER_FLAGS="--prefer-lowest"
+
+before_install:
+    - travis_retry composer self-update
+
+install:
+    - composer update ${COMPOSER_FLAGS} --no-interaction