added require-dev block for tests, added proper description for suggested package

composer.json

@@ -15,9 +15,12 @@
   "require": {
     "php": ">=5.3.0"
   },
-  "suggest": {
+  "require-dev": {
     "mlehner/gelf-php": "dev-master"
   },
+  "suggest": {
+    "mlehner/gelf-php": "Allow sending log messages to a GrayLog2 server"
+  },
   "autoload": {
     "psr-0": {"Monolog": "src/"}
   }