Remove unnecessary "," from Pylint version regex

Summary:
See PHI1238. This "," in the regex can only make the lint binding fail if there is no "," in the version output string. In modern versions of Pylint, there is (apparently) no comma in the version string. Remove it.

See also https://discourse.phabricator-community.org/t/arcanistpylintlinter-version-regex-issue/2688

Test Plan:

$ pip install pylint
Traceback (most recent call last):
  File "/Users/epriestley/Library/Python/2.7/bin/pip", line 6, in <module>
    from pip._internal import main
ImportError: No module named pip._internal

¯\_(ツ)_/¯

Reviewers: amckinley, joshuaspence

Reviewed By: joshuaspence

Differential Revision: https://secure.phabricator.com/D20505