espota: fixed remaining python 3.5 incompatibility

[asdwsda]

Sadly I didn't use authentication for ArduinoOTA so far. When I tried it turned out that it doesn't work with python 3.5...

Traceback (most recent call last):
  File "/home/bitn/.arduino15/packages/esp8266/hardware/esp8266/2.0.0/tools/espota.py", line 318, in <module>
    sys.exit(main(sys.argv))
  File "/home/bitn/.arduino15/packages/esp8266/hardware/esp8266/2.0.0/tools/espota.py", line 313, in main
    return serve(options.esp_ip, options.host_ip, options.esp_port, options.host_port, options.auth, options.image, command)
  File "/home/bitn/.arduino15/packages/esp8266/hardware/esp8266/2.0.0/tools/espota.py", line 107, in serve
    cnonce = hashlib.md5(cnonce_text).hexdigest()
TypeError: Unicode-objects must be encoded before hashing

Fixed the error and tested with python 2.7.11 and 3.5.1 as before. Everything worked fine with all the parameters, so I hope these were the last ones.
Sorry for the double pull request.