feature #1067 Validate composer files while TravisCI build (voronkovich)

This PR was squashed before being merged into the master branch (closes #1067).

Discussion
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Validate composer files while TravisCI build

This check prevents us from having an out of sync composer's lock-file (For example, when someone forgot to commit composer.lock file: #1066).

Commits
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8438677 Validate composer files while TravisCI build

Author: javiereguiluz

.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