Travis CI should use bin/composer and not system-wide Composer.

.travis.yml

@@ -9,7 +9,7 @@ php:
 before_script:
     - sudo apt-get install parallel
     - echo '' > ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini
-    - composer install --dev --prefer-source
+    - bin/composer install --dev --prefer-source
     - git config --global user.name travis-ci
     - git config --global user.email travis@example.com