php: update composer.json
diff --git a/composer.json b/composer.json
index 0ebe0a1..7836620 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,7 @@
   "license": "BSD-3-Clause",
   "require": {
     "php": ">=5.5.0",
-    "stanley-cheung/protobuf-php": "dev-master"
+    "stanley-cheung/protobuf-php": "v0.6"
   },
   "require-dev": {
     "google/auth": "v0.9"