Change dev-master to 6.x

Author: philipobenito

composer.json

@@ -57,7 +57,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "5.x-dev",
+            "dev-master": "6.x-dev",
             "dev-6.x": "6.x-dev",
             "dev-5.x": "5.x-dev",
             "dev-4.x": "4.x-dev",