Fix TypeError

[fgervais]

Traceback (most recent call last):
...
File "python-adb/adb/sign_pythonrsa.py", line 30, in update
self._buf += msg
TypeError: Can't convert 'bytes' object to str implicitly

[googlebot]

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[fahhem]

Are you running with Python 2 or 3?

[coveralls]

Coverage remained the same at 42.384% when pulling e7d0b2d on fgervais:type-error into ee2d54e on google:master.

[coveralls]

Coverage remained the same at 42.384% when pulling e7d0b2d on fgervais:type-error into ee2d54e on google:master.

[fgervais]

Python 3.5.2

[googlebot]

CLAs look good, thanks!