Use --prefer-source for php-coveralls installation

Author: jsor

.travis.yml

@@ -14,7 +14,7 @@ before_install:
 
 install:
   - composer install --prefer-source
-  - composer require satooshi/php-coveralls:dev-master
+  - composer require satooshi/php-coveralls:dev-master --prefer-source
 
 script:
   - phpunit -v --coverage-text --coverage-clover ./build/logs/clover.xml