Clear os.walk's dirnames to properly exclude dirs

I have a project:
`./foo/__init__.py`
`./foo/bar/__init__.py`
`./foo/baz.py`
`./__init__.py`
`./qux.py`

Running: `flake8 --exclude="foo" ./`

Checks:
`./foo/bar/__init__.py`
`./__init__.py`
`./qux.py`

This patch prevents the continued walking of **foo**'s subdirectories and not
just files.