Switch to the docker-based infrastructure on Travis

.travis.yml

@@ -1,5 +1,15 @@
 language: php
 
+sudo: false
+
+cache:
+  directories:
+    - $HOME/.composer/cache
+
+addons:
+  apt_packages:
+    - parallel
+
 php:
   - 5.3.3
   - 5.3
@@ -10,10 +20,9 @@ php:
   - hhvm
 
 before_script:
-    - sudo apt-get install parallel
     - rm -f ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini
-    - composer install --prefer-source
-    - bin/composer install --prefer-source
+    - composer install
+    - bin/composer install
     - git config --global user.name travis-ci
     - git config --global user.email travis@example.com