Update graceful-fs

At 3.x, `graceful-fs` should only be used where it is explicitly `require`d (see https://github.com/isaacs/node-graceful-fs/commit/08471b246bec73848b53e9ad4972f5fe02031805).  Updating in an effort to address [transient build failures](https://travis-ci.org/openlayers/closure-util/jobs/27319389).