Validate composer files while TravisCI build

Author: voronkovich

.travis.yml

@@ -57,3 +57,4 @@ script:
   - ./bin/console doctrine:schema:validate --skip-sync -vvv --no-interaction
   #  Fail CI if the repo is in a dirty state after building assets (only for current release ie install)
   #-  if [[ "$ACTION" == "install" ]]; then yarn install && yarn encore production && git add --all && git diff --staged --exit-code; fi
+  - composer validate