telegram_webhooks doesn't crash when bot's token is nil

Fixes #11

lib/telegram/bot/routes_helper.rb

@@ -20,7 +20,7 @@ module Telegram
         # Replaces colon with underscore so rails don't treat it as
         # route parameter.
         def escape_token(token)
-          token.tr(':', '_')
+          token && token.tr(':', '_')
         end
       end