Travis: disallow PEP-8 failures from now on
Fix __str__ method on the TimeoutExpired exception class. Also fix argument
order in the class constructor so it uses the same order as the actual class
from Python 3.3 and above.

TimeoutExpired from Python 3.3 takes arguments in the following order:
cmd, timeout, output.

See https://github.com/python/cpython/blob/master/Lib/subprocess.py#L388