Relax the version regex for ArcanistFlake8Linter.

Summary: As noticed by @epriestley in D9060, flake8 --version can emit a string like "2.0" rather than the expected "2.0.0".

Test Plan: Uninstalled my existing version of flake8 (v2.1.0) and installed a prior version (pip install flake8==2.0.0). Ran flake8 --version and inspected the output.

Reviewers: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D9174