Check if currentDir is readable on loop iteration when initializing Config

[MasterOdin]

Fixes #2549

is_readable will return false if the passed directory falls outside of a set open_basedir php.ini directive, while also raising a warning that is suppressed here. I assume that phpcs is initially run from a directory inside the open_basedir directive. I wasn't sure if/how one might add tests for this.

[gsherwood]

Thanks a lot for this PR. The fix worked great.