remove mkdirp use

node 10.12 has { recursive: true }

src/storage/filesystem.js

@@ -30,7 +30,6 @@ var assert = require('assert'),
     df = require('@sindresorhus/df'),
     EventEmitter = require('events'),
     fs = require('fs'),
-    mkdirp = require('mkdirp'),
     path = require('path'),
     prettyBytes = require('pretty-bytes'),
     readdirp = require('readdirp'),
@@ -86,7 +85,7 @@ function upload(apiConfig, backupFilePath, sourceStream, callback) {
     assert.strictEqual(typeof sourceStream, 'object');
     assert.strictEqual(typeof callback, 'function');
 
-    mkdirp(path.dirname(backupFilePath), function (error) {
+    fs.mkdir(path.dirname(backupFilePath), { recursive: true }, function (error) {
         if (error) return callback(new BoxError(BoxError.EXTERNAL_ERROR, error.message));
 
         safe.fs.unlinkSync(backupFilePath); // remove any hardlink
@@ -168,7 +167,7 @@ function copy(apiConfig, oldFilePath, newFilePath) {
 
     var events = new EventEmitter();
 
-    mkdirp(path.dirname(newFilePath), function (error) {
+    fs.mkdir(path.dirname(newFilePath), { recursive: true }, function (error) {
         if (error) return events.emit('done', new BoxError(BoxError.EXTERNAL_ERROR, error.message));
 
         events.emit('progress', `Copying ${oldFilePath} to ${newFilePath}`);