Lock ergebnis/composer-normalize to v2.9.0

v2.9.1 of the plugin dropped support for Composer v1, see ergebnis/composer-normalize#597

Author: pierlon

.travis.yml

@@ -69,7 +69,8 @@ jobs:
       env: WP_VERSION=latest DEV_LIB_ONLY=phpsyntax
       before_script:
         - phpenv config-rm xdebug.ini || echo "xdebug.ini does not exist."
-        - composer require --dev localheinz/composer-normalize --ignore-platform-reqs
+        # TODO: Exact version of ergebnis/composer-normalize can be removed once Composer v2 is supported.
+        - composer require --dev ergebnis/composer-normalize:2.9.0 --ignore-platform-reqs
       script:
         - source "$DEV_LIB_PATH/travis.script.sh"
         - composer validate --no-check-all