Update .travis.yml

Author: emonkak

.travis.yml

@@ -12,11 +12,13 @@ cache:
     - $HOME/.composer/cache
 
 before_script:
-  - composer install
+  - composer install --prefer-source
 
 script:
   - mkdir -p build/logs
   - phpunit --coverage-clover build/logs/clover.xml
 
 after_script:
   - php vendor/bin/coveralls
+
+sudo: false