Remove external df module

It has some parsing issues with locale

src/storage/filesystem.js

@@ -35,7 +35,7 @@ const assert = require('assert'),
     BoxError = require('../boxerror.js'),
     constants = require('../constants.js'),
     debug = require('debug')('box:storage/filesystem'),
-    df = require('@sindresorhus/df'),
+    df = require('../df.js'),
     fs = require('fs'),
     mounts = require('../mounts.js'),
     path = require('path'),