use composer install with dist for faster endor's download

.travis.yml

@@ -13,7 +13,7 @@ install:
   - ./elasticsearch-${ES_VERSION}/bin/elasticsearch -d
 before_script:
   - composer config -g github-oauth.github.com $GITHUB_COMPOSER_AUTH
-  - composer install --no-interaction
+  - composer install --no-interaction --prefer-dist
 script:
   - wget -q --waitretry=1 --retry-connrefused -T 10 -O - http://127.0.0.1:9200
   - vendor/bin/phpunit --coverage-clover=coverage.clover