build: Force composer 1.x in Travis CI

Bug: T266417
Change-Id: If4a21e46fc8d43336a11cc75147592ae6c4ebd5a

Author: reedy

.travis.yml

@@ -48,6 +48,8 @@ addons:
 
 before_script:
   - echo 'opcache.enable_cli = 1' >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
+  # Force composer 1.x because MediaWiki/composer-merge-plugin don't support composer 2.x yet
+  - composer self-update --1
   - composer install --prefer-source --quiet --no-interaction
   # At Travis CI, the mysql user 'travis' doesn't have create database rights, use 'root' instead.
   - >