use cache for composer dependencies in travis
use cache for composer dependencies in travis

sphereio/sphere-php-sdk,sphereio/commercetools-php-sdk,sphereio/commercetools-php-sdk,sphereio/commercetools-php-sdk,sphereio/commercetools-php-sdk,sphereio/sphere-php-sdk,sphereio/sphere-php-sdk,sphereio/sphere-php-sdk