Updating PEP8 1.5.6 from 1.0.1

The one obvious blocking change is that PEP8 changed how they process options
in version 1.3 around a StyleGuide object. This changed the internal api which
Khan Linter uses.

See:
https://github.com/jcrocholl/pep8/blob/96290d48b1334fe8542666174d7775cda3f46d9a/CHANGES.txt#L343

Currently, if you accidentally use PEP 1.3+ with Khan Linter, you'll get your
results, but _DEFAULT_PEP8_ARGS will be ignored.

Reading through the rest of the changelog and quick informal tests imply that
this is the only internal api change and upgrading is otherwise safe.
(Famous last words...)

Just for reference:
1.0.1 was released 2012-04-06
1.5.6 was released 2014-04-14