make espota compatible with python 3.5

[asdwsda]

With python 3.5 (default on Arch linux) I got the following:

Traceback (most recent call last):
  File "espota.py", line 318, in <module>
    sys.exit(main(sys.argv))
  File "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 "espota.py", line 95, in serve
    sent = sock2.sendto(message, remote_address)
TypeError: a bytes-like object is required, not 'str'

This patch will fix the error. Tested with python 2.7.11 and 3.5.1.