Allow Symfony 4 dependencies.

Author: bangpound

composer.json

@@ -11,11 +11,11 @@
     },
     "require": {
         "bangpound/composer-constants": "^1.0",
-        "symfony/config": "~2.8|~3.0",
-        "symfony/dependency-injection": "~2.8|~3.0",
-        "symfony/expression-language": "~2.8|~3.0",
-        "symfony/http-kernel": "~2.8|~3.0",
-        "symfony/yaml": "~2.8|~3.0"
+        "symfony/config": "~2.8|~3.0|~4.0",
+        "symfony/dependency-injection": "~2.8|~3.0|~4.0",
+        "symfony/expression-language": "~2.8|~3.0|~4.0",
+        "symfony/http-kernel": "~2.8|~3.0|~4.0",
+        "symfony/yaml": "~2.8|~3.0|~4.0"
     },
     "replace": {
         "drupal/componser_manager": "*"