Allow upgrading to Symfony 4

The test suite says it is compatible.

composer.json

@@ -19,15 +19,15 @@
         "php": "^5.6 || ^7.0",
         "cocur/slugify": "^1.4 || ^2.0 || ^3.0",
         "sonata-project/datagrid-bundle": "^2.0",
-        "symfony/config": "^2.8 || ^3.2",
-        "symfony/form": "^2.8 || ^3.2",
-        "symfony/framework-bundle": "^2.8 || ^3.2",
-        "symfony/http-foundation": "^2.8 || ^3.2",
-        "symfony/property-access": "^2.8 || ^3.2",
-        "symfony/security": "^2.8 || ^3.2",
-        "symfony/translation": "^2.8 || ^3.2",
-        "symfony/twig-bridge": "^2.8 || ^3.2",
-        "symfony/validator": "^2.8 || ^3.2",
+        "symfony/config": "^2.8 || ^3.2 || ^4.0",
+        "symfony/form": "^2.8 || ^3.2 || ^4.0",
+        "symfony/framework-bundle": "^2.8 || ^3.2 || ^4.0",
+        "symfony/http-foundation": "^2.8 || ^3.2 || ^4.0",
+        "symfony/property-access": "^2.8 || ^3.2 || ^4.0",
+        "symfony/security": "^2.8 || ^3.2 || ^4.0",
+        "symfony/translation": "^2.8 || ^3.2 || ^4.0",
+        "symfony/twig-bridge": "^2.8 || ^3.2 || ^4.0",
+        "symfony/validator": "^2.8 || ^3.2 || ^4.0",
         "twig/extensions": "^1.5",
         "twig/twig": "^1.34 || ^2.0"
     },
@@ -43,7 +43,7 @@
         "sensio/framework-extra-bundle": "^3.0 || ^4.0 || ^5.0",
         "sllh/php-cs-fixer-styleci-bridge": "^2.0",
         "sonata-project/exporter": "^1.3",
-        "symfony/phpunit-bridge": "^3.3.12"
+        "symfony/phpunit-bridge": "^3.3.12 || ^4.0"
     },
     "autoload": {
         "psr-4": { "Sonata\\CoreBundle\\": "src/" }