TypeError: not enough arguments for format string in airpurifier.py

[sq5gvm]

Commit 65aeca5 that added button_pressed property introduced two errors, one of which results in module not working and returning error messages as:

Traceback (most recent call last):
  File "./MyAir.py", line 64, in <module>
    print(MyAir.status())
  File "/usr/local/lib/python3.5/dist-packages/miio/airpurifier.py", line 322, in __repr__
    "button_pressed=%s>" % \
TypeError: not enough arguments for format string

Merge request on its way.