build(deps): upgrade mkdirp to 0.5.4 for security

This is actually a security issue in minimist, which is not actually
used in this module. This module only uses the API of mkdirp, not the
CLI (which is where minimist is used). So this is really a red herring.

Closes #85.
Closes #86.

Author: malept

Diff for: package.json

@@ -20,7 +20,7 @@
   "dependencies": {
     "concat-stream": "1.6.2",
     "debug": "2.6.9",
-    "mkdirp": "0.5.1",
+    "mkdirp": "0.5.4",
     "yauzl": "2.4.1"
   },
   "devDependencies": {