make sure stable dependencies versions will be installed

Author: saimaz

Diff for: .travis.yml

@@ -14,7 +14,7 @@ install:
   - ./elasticsearch-${ES_VERSION}/bin/elasticsearch -d
 before_script:
   - if [ "$GITHUB_COMPOSER_AUTH" ]; then composer config -g github-oauth.github.com $GITHUB_COMPOSER_AUTH; fi
-  - composer install --no-interaction --prefer-stable
+  - composer update --no-interaction --prefer-stable
 script:
   - wget -q --waitretry=1 --retry-connrefused -T 10 -O - http://127.0.0.1:9200
   - vendor/bin/phpunit --coverage-clover=coverage.xml