Add a getVersion function to ArcanistExternalLinter.

Summary:
This method will, theoretically, allow arc lint to be configured to require some minimum version of an external linter (although this would probably require significantly more work).

Additionally, the existence of this method simplifies the getCacheVersion function which, previously, was implemented by the external linters individually. Instead, a general approach to determining the version for cacheing purposes can be used.

Fixes T4954.

Test Plan: I'm not sure how to test this.

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4954

Differential Revision: https://secure.phabricator.com/D8971