Fix parsing of ruby version for ArcanistRubyLinter

Summary:
This fails to extract the version correctly locally.

> ruby --version
ruby 1.8.7 (2014-01-28 patchlevel 376) [x86_64-linux]

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14525