Update branch metadata

We are moving from the branch name "master".

Author: greg0ire

.doctrine-project.json

@@ -5,7 +5,7 @@
   "versions": [
     {
       "name": "3.1",
-      "branchName": "master",
+      "branchName": "3.1.x",
       "slug": "3.1",
       "upcoming": true
     },