Allow to use symfony 4.0 (#75)

* Allow to use symfony 4.0
* Consistent whitespace

Author: Lars Roettig

composer.json

@@ -7,7 +7,7 @@
         "php": ">= 7.1",
         "mustache/mustache": "^2.12",
         "ralouphie/mimey": "^2.0",
-        "symfony/yaml": "^2.3||^3.3",
+        "symfony/yaml": "^2.3 || ^3.3 || ^4.0",
         "zendframework/zend-http": "^2.6",
         "zendframework/zend-stdlib": "^2.7 || ^3.2"
     },