Don't explicitly use an interpreter for pep8

Summary: Depends on D9430. If we are using the system installation of pep8, then it probably unnecessary (and possibly wrong) to specify python2.6 as the default interpreter.

Test Plan: Ran arc unit.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: vrusinov, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9474