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