Added --prefer-source flag to composer install command

- GitHub now wants zipball downloads to go through the API, which quickly runs
  into limits. Adding the --prefer-source flag circumvents that at the expense
  of a costlier clone operation.

.travis.yml

@@ -7,7 +7,7 @@ php:
 
 before_install:
  - cp tests/TestConfiguration.php.travis tests/TestConfiguration.php
- - composer install --dev
+ - composer install --dev --prefer-source
  - wget http://cs.sensiolabs.org/get/php-cs-fixer.phar
 
 script: