Update requirements

composer.json

@@ -16,7 +16,7 @@
     "php": ">=5.3.0"
   },
   "require-dev": {
-    "mlehner/gelf-php": "dev-master"
+    "mlehner/gelf-php": "1.0.*"
   },
   "suggest": {
     "mlehner/gelf-php": "Allow sending log messages to a GrayLog2 server"