update composer before starting composer install

.travis.yml

@@ -8,7 +8,7 @@ env:
   - DB_USER=root
 
 before_script:
+  - composer self-update
   - composer install --prefer-dist --dev
   - sh -c "mysql -u$DB_USER -e 'SET FOREIGN_KEY_CHECKS = 0; DROP DATABASE IF EXISTS thelia;SET FOREIGN_KEY_CHECKS = 1;'; fi"
   - php Thelia thelia:install --db_host=localhost --db_username=$DB_USER --db_name=thelia
-  - composer self-update