Update branch alias from 3.6-dev to 3.7-dev

Author: barryvdh

composer.json

@@ -66,7 +66,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "3.6-dev"
+            "dev-master": "3.7-dev"
         },
         "laravel": {
             "providers": [