Fixed regressions of b0cd246 and 3128fdf

3128fdf:
- Secret can be a string or function
- Compatibility with older node versions

b0cd246:
- Fixed dependencies

lib/index.js

@@ -111,9 +111,9 @@ function noQsMethod(options) {
 
 function authorize(options, onConnection) {
   options = xtend({ decodedPropertyName: 'decoded_token', encodedPropertyName: 'encoded_token' }, options);
-  
-  if (typeof options.secret !== 'string') {
-    throw new Error(`Provided secret "${options.secret}" is invalid, must be of type string.`)
+
+  if (typeof options.secret !== 'string' && typeof options.secret !== 'function') {
+    throw new Error('Provided secret "' + options.secret + '" is invalid, must be of type string or function.')
   }
 
   if (!options.handshake) {