Subversion - set minimum required version to 1.5

Summary: Fixes T7228.

Test Plan: hacked $version to be '1.4' and saw the proper error message

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7228

Differential Revision: https://secure.phabricator.com/D11732