minor #13 add Symfony 4 support (xabbuh)

This PR was merged into the 2.1.x-dev branch.

Discussion
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add Symfony 4 support

Commits
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6bc6bc7 add Symfony 4 support

Author: xabbuh

composer.json

@@ -16,7 +16,7 @@
         "php-xapi/exception": "^0.2.0",
         "php-xapi/model": "^1.2 || ^2.1 || ^3.0",
         "php-xapi/serializer": "^2.1.1",
-        "symfony/serializer": "~2.8|~3.0"
+        "symfony/serializer": "^2.8 || ^3.4 || ^4.0"
     },
     "require-dev": {
         "php-xapi/json-test-fixtures": "^2.0",