use composer install from travis container to ensure travis can remove files after build

Author: tschortsch

.travis.yml

@@ -17,9 +17,9 @@ before_install:
   - nvm install 12
 
 install:
+  - composer install
   - npm ci
   - scripts/install-wp.sh
-  - npm run env docker-run -- php composer install
 
 script:
   - npm run lint