Make sure composer files are valid

The composer.lock is put under version control and it often happens to
be out of sync with the composer.json, which could lead to
hard-to-understand issues.
Using the --strict option here because we might as well aim for a
perfectly valid composer.json

.travis.yml

@@ -18,7 +18,7 @@ before_install:
   - mv ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini{,.disabled} || echo "xdebug not available"
   - composer self-update
 
-install: travis_retry composer update --prefer-dist
+install: travis_retry composer validate --strict && composer update --prefer-dist
 
 script:
   - if [[ "$DB" == "mysql" || "$DB" == "mariadb" ]]; then mysql -e "CREATE SCHEMA doctrine_tests; GRANT ALL PRIVILEGES ON doctrine_tests.* to travis@'%'"; fi