Revert "Use Composer from build/tools/composer"

This reverts commit dcc3675.

Author: sebastianbergmann

.travis.yml

@@ -9,8 +9,11 @@ php:
 
 sudo: false
 
+before_install:
+  - composer self-update
+
 install:
-  - travis_retry ./build/tools/composer install --no-interaction --prefer-source
+  - travis_retry composer install --no-interaction --prefer-source
 
 script:
   - ./phpunit