Mark 4.x as deployable branch (#169)

Author: keelerm84

.ldrelease/config.yml

@@ -8,6 +8,11 @@ publications:
   - url: https://packagist.org/packages/launchdarkly/server-sdk
     description: Packagist
 
+branches:
+  - name: main
+    description: 5.x
+  - name: 4.x
+
 jobs:
   - template:
       name: php