Update travis configuration to use container based infrastructure, also use...

Update travis configuration to use container based infrastructure, also use cache for composer in order to reduce network usage

.travis.yml

 language: php
 
+sudo: false
+
+cache:
+    directories:
+      - $HOME/.composer/cache
+
 before_script:
     # symfony/*
     - sh -c "if [ '$SYMFONY_DEPS_VERSION' = '2.8' ]; then sed -i 's/~2.3/2.8.*@dev/g' composer.json; composer update --dev --prefer-source; fi"
@@ -10,21 +16,16 @@ before_script:
 
 script: phpunit
 
-php:
-  - 5.3
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7.0
-  - hhvm
-
 matrix:
-    allow_failures:
-        - php: hhvm
     include:
+        - php: 5.3
+        - php: 5.4
+        - php: 5.5
         - php: 5.6
           env: SYMFONY_DEPS_VERSION=2.3
         - php: 5.6
           env: SYMFONY_DEPS_VERSION=2.7
         - php: 5.6
           env: SYMFONY_DEPS_VERSION=2.8
+        - php: 7.0
+        - php: hhvm