composer.json: Add 2.1.x-dev alias for pico-2.1 branch

composer.json

@@ -52,6 +52,7 @@
     "extra": {
         "branch-alias": {
             "dev-master": "2.0.x-dev",
+            "dev-pico-2.1": "2.1.x-dev",
             "dev-pico-3.0": "3.0.x-dev"
         }
     }