MQTT connection failed

## Description of the problem
When trying to run the echo bot example I'm getting:
fbchat._exception.FBchatException: MQTT connection failed

## Code to reproduce
https://fbchat.readthedocs.io/en/stable/examples.html#echobot

## Traceback
```
During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/peter/PycharmProjects/shrek_messenger/echo.py", line 22, in
client.listen()
File "/Users/peter/PycharmProjects/shrek_messenger/venv/lib/python3.7/site-packages/fbchat/_client.py", line 2883, in listen
self.startListening()
File "/Users/peter/PycharmProjects/shrek_messenger/venv/lib/python3.7/site-packages/fbchat/_client.py", line 2835, in startListening
foreground=True,
File "/Users/peter/PycharmProjects/shrek_messenger/venv/lib/python3.7/site-packages/fbchat/_mqtt.py", line 65, in connect
raise _exception.FBchatException("MQTT connection failed")
fbchat._exception.FBchatException: MQTT connection failed
```