Remove default master key.

If there is a default master key, people will inevitably use it. And that would be bad.

README.md

@@ -58,7 +58,7 @@ var api = new ParseServer({
   databaseURI: 'mongodb://localhost:27017/dev',
   cloud: '/home/myApp/cloud/main.js', // Provide an absolute path
   appId: 'myAppId',
-  masterKey: 'mySecretMasterKey',
+  masterKey: '', //Add your master key here. Keep it secret!
   fileKey: 'optionalFileKey',
   serverURL: 'http://localhost:' + port + '/parse' // Don't forget to change to https if needed
 });