minor #669 Use the new "json" type from DBAL 2.6 (javiereguiluz)

This PR was merged into the master branch.

Discussion
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Use the new "json" type from DBAL 2.6

This partially fixes #620 and it's based on #659.

Commits
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189083f Use the new "json" type from DBAL 2.6

src/Entity/User.php

@@ -69,7 +69,7 @@ class User implements UserInterface, \Serializable
     /**
      * @var array
      *
-     * @ORM\Column(type="json_array")
+     * @ORM\Column(type="json")
      */
     private $roles = [];