rfkill_unblock attempts to write bytes type to file opened with 'w' flag

[rfkill_unblock](https://github.com/wraith-wireless/PyRIC/blob/master/pyric/utils/rfkill.py#L133) misses a conversion to str type returned from rfkill_event (returns struct.pack()) which is bytes in python3.

Recommend implementing the check in [rfkill_block](https://github.com/wraith-wireless/PyRIC/blob/master/pyric/utils/rfkill.py#L103) rather than write to file using 'wb' flags.