Update Adafruit_MQTT_Library

[amotl]

Hi there,

the Adafruit_MQTT_Library is available as version 2.4.2 already. However, some of the sketches here are still using version 1.0.3 on behalf of our fork [1]. We might consider updating it to the most recent version.

With kind regards,
Andreas.

[1] https://github.com/daq-tools/Adafruit_MQTT_Library