Update symfony 4 compatibility

.travis.yml

@@ -33,6 +33,8 @@ matrix:
       # bleeding edge (unreleased dev versions where failures are allowed)
       - php: 7.1
         env: SYMFONY_VERSION="3.4.*"
+      - php: 7.1
+        env: SYMFONY_VERSION="4.0.*"
       - php: nightly
         env: SYMFONY_VERSION="4.0.*"
     allow_failures:
@@ -46,7 +48,7 @@ cache:
 before_install:
   - if [[ "$ENABLE_CODE_COVERAGE" != "yes" ]]; then phpenv config-rm xdebug.ini || true; fi;
   - if [[ "$SYMFONY_VERSION" != "" ]]; then composer require "symfony/symfony:${SYMFONY_VERSION}" --no-update; fi;
-  - if [[ "$TRAVIS_PHP_VERSION" == "nightly" ]]; then composer remove "friendsofphp/php-cs-fixer" --no-update --no-interaction --dev; fi;
+  - if [[ "$CHECK_PHP_SYNTAX" != "yes" ]]; then composer remove "friendsofphp/php-cs-fixer" --no-update --no-interaction --dev; fi;
 
 install:
   - composer update --prefer-dist --no-interaction --optimize-autoloader --prefer-stable --no-progress $COMPOSER_FLAGS