Switch to a more supported version of PubSubClient

[thegreatgunbantoad]

Is it worth switching over to the more upto date: https://github.com/hmueller01/pubsubclient3 (At a minimum it resolves a single compiler warning)

I think just changing line 23 in platformio.ini should do it:
PubSubClient3

Would be nice if someone using MQTT could see if this works.