Simplify version parsing for ArcanistPhpLinter

Summary: We can use php --run "echo phpversion();" to return a nice, compact representation of the PHP version.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11264