Relax the version regex for `ArcanistFlake8Linter`.
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

PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,KieranHayes/arcanist,PageUpPeopleOrg/arcanist-patches,sneakpeeq/arcanist,calblueprint/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,coursera/arcanist,wharris/arcanist,phacility/arcanist,alexmv/arcanist,phacility/arcanist,fivestars/arcanist,KieranHayes/arcanist,eSpark/arcanist,wxstars/arcanist,chundongwang/arcanist,calblueprint/arcanist,aik099/arcanist,kevinseelbach/arcanist,eSpark/arcanist,coursera/arcanist,codelabs-ch/arcanist,alexmv/arcanist,Symplicity/arcanist,Symplicity/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,haskell-infra/arcanist,wikimedia/phabricator-arcanist,eSpark/arcanist,kevinseelbach/arcanist,aik099/arcanist,codelabs-ch/arcanist,aik099/arcanist,folsom-labs/arcanist,memsql/arcanist,wharris/arcanist,memsql/arcanist,codelabs-ch/arcanist,uber/arcanist,kevinseelbach/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,zhihu/arcanist,codelabs-ch/arcanist,hach-que/arcanist,chundongwang/arcanist,chundongwang/arcanist,KieranHayes/arcanist,wxstars/arcanist,phacility/arcanist,alexmv/arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,phacility/arcanist,Khan/arcanist,zhihu/arcanist,Khan/arcanist,hach-que/arcanist,jra3/arcanist,coursera/arcanist,kevinseelbach/arcanist,r4nt/arcanist,sneakpeeq/arcanist,calblueprint/arcanist,calblueprint/arcanist,uber/arcanist,folsom-labs/arcanist,folsom-labs/arcanist,aik099/arcanist,jra3/arcanist,fivestars/arcanist,jra3/arcanist,wxstars/arcanist,phacility/arcanist,uber/arcanist,memsql/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,haskell-infra/arcanist,wharris/arcanist,Khan/arcanist,eSpark/arcanist,coursera/arcanist,r4nt/arcanist,KieranHayes/arcanist,zhihu/arcanist,chundongwang/arcanist,dguo-coursera/arcanist,uber/arcanist,haskell-infra/arcanist,Khan/arcanist,dguo-coursera/arcanist,memsql/arcanist,Symplicity/arcanist,r4nt/arcanist,sneakpeeq/arcanist,zhihu/arcanist,folsom-labs/arcanist,Symplicity/arcanist,sneakpeeq/arcanist,hach-que/arcanist,fivestars/arcanist,hach-que/arcanist,fivestars/arcanist,wharris/arcanist,alexmv/arcanist,PageUpPeopleOrg/arcanist-patches