Update travis build file : add --prefer-source to composer install, add build on PHP 5.5

.travis.yml

@@ -4,12 +4,13 @@ before_script:
   - sudo apt-get update
   - sudo easy_install sphinx
   - sudo apt-get install -y ffmpeg libavcodec-extra-53
-  - curl -s http://getcomposer.org/installer | php
-  - php composer.phar install --dev
+  - composer self-update
+  - composer install --dev --prefer-source
 
 php:
   - 5.3
   - 5.4
+  - 5.5
 
 script:
   - phpunit