Change the mode of the directory created by `PhutilKeyValueCacheDirectory`

Summary: Change the mode of the directory created by `PhutilKeyValueCacheDirectory` from `0777` to `0755`. In general, this directory //shouldn't// need `g+x` or `o+x` permissions.

Test Plan: Ran `diviner generate` and checked the permissions on the `.divinercache` directory.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9947