Update project to Symfony 4 beta4

Author: javiereguiluz

.travis.yml

@@ -32,7 +32,7 @@ script:
   # this checks that the Twig template files contain no syntax errors
   - ./bin/console lint:twig templates
   # this checks that the XLIFF translations contain no syntax errors
-  - ./bin/console lint:xliff translations
+  - ./bin/console lint:xliff config/translations
   # this checks that the application doesn't use dependencies with known security vulnerabilities
   - ./bin/console security:check --end-point=http://security.sensiolabs.org/check_lock
   # this checks that the composer.json and composer.lock files are valid