Revert "Fix ES6 syntax errors"

This reverts commit 0ed1efedfb.

src/telegramWebHook.js

@@ -45,7 +45,7 @@ class TelegramBotWebHook {
   }
 
   // pipe+parse body
-  _parseBody(err, body) {
+  _parseBody = (err, body) => {
     if (err) {
       return debug(err);
     }
@@ -59,7 +59,7 @@ class TelegramBotWebHook {
   }
 
   // bound req listener
-  _requestListener(req, res) {
+  _requestListener = (req, res) => {
     debug('WebHook request URL:', req.url);
     debug('WebHook request headers: %j', req.headers);