Fix for Ethernet FeatherWing

This version removes the archived Adafruit Ethernet2 library as a dependency and switches to the actively maintained Arduino Ethernet Library (https://github.com/arduino-libraries/Ethernet).

Changelog:

• Adafruit Ethernet 2 -> Arduino Ethernet.
• Updated AdafruitIO_Ethernet.h to include a check for if the shield is unplugged.
• Switch MQTT Ethernet clients to only connect on port 1883 while retaining _mqtt_port as 8883 for all WiFi MQTT connections.
• Explicitly pass MQTTClient the Adafruit IO Username and Key.
• Updated project dependencies to reflect the switch to Arduino Ethernet.
• Updated documentation and links.
• Updated travis configuration to remove slowdowns caused by testing sensor-dependent sketches, only testing the core API sketches.
• Updated README to include new TravisCI build link.