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