Bump branch alias to 2.4.

Author: Maxime Rainville

composer.json

@@ -21,7 +21,11 @@
     "require-dev": {
         "phpunit/phpunit": "^5.7"
     },
-    "extra": { },
+    "extra": {
+        "branch-alias": {
+            "2.x-dev": "2.4.x-dev"
+        }
+    },
     "autoload": {
         "psr-4": {
             "SilverStripe\\PostgreSQL\\": "code/",