minor #718 Adds Doctrine's mapping validation on Travis. (phpeek)

This PR was merged into the master branch.

Discussion
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Adds Doctrine's mapping validation on Travis.

Runs `doctrine:schema:validate` command with database sync skipping and
no interaction flags.

This is what I usually setup in CI for my projects to avoid common Doctrine's mapping errors e.g. missing `inversedBy` properties.

Commits
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2faf341 Adds Doctrine's mapping validation on Travis.

.travis.yml

@@ -41,3 +41,5 @@ script:
   - ./bin/console security:check --end-point=http://security.sensiolabs.org/check_lock
   # this checks that the composer.json and composer.lock files are valid
   - composer validate --strict
+  # this checks that Doctrine's mapping configurations are valid
+  - ./bin/console doctrine:schema:validate --skip-sync -vvv --no-interaction