Updates branch aliases

Updates branch aliases to match recommendations from Composer docs:

- dev-master => 2.6.x-dev
- dev-develop => 2.7.x-dev

Author: weierophinney

composer.json

@@ -40,8 +40,8 @@
     "prefer-stable": true,
     "extra": {
         "branch-alias": {
-            "dev-master": "2.6-dev",
-            "dev-develop": "2.7-dev"
+            "dev-master": "2.6.x-dev",
+            "dev-develop": "2.7.x-dev"
         }
     },
     "scripts": {