Fix the usage of the composer cache on Travis

Author: stof

.travis.yml

@@ -30,7 +30,7 @@ matrix:
 before_script:
   - sh bin/run-"$WEBDRIVER".sh
 
-  - composer install --prefer-source
+  - composer install
 
   # Start a webserver for web fixtures. Force using PHP 5.6 to be able to run it on PHP 5.3 and HHVM jobs too
   - MINK_PHP_BIN=~/.phpenv/versions/5.6/bin/php vendor/bin/mink-test-server > /dev/null 2>&1 &