Prefer stable dependencies for development

See https://getcomposer.org/doc/04-schema.md#prefer-stable

Author: Steveb-p

composer.json

@@ -108,6 +108,7 @@
       "ext-mongodb": "1.3",
       "ext-bcmath": "1",
       "ext-mbstring": "1"
-    }
+    },
+    "prefer-stable": true
   }
 }