update mkdirp (to not use old version of minimist with security vulnerabilities)

[gharlan]

modernizr requires mkdirp 0.5.1.
And mkdirp 0.5.1 requires minimist 0.0.8, which has a security vulnerability: https://snyk.io/vuln/SNYK-JS-MINIMIST-559764

[rejas]

This dependency wont get updated until the next major version, see #2499 on why I reverted that update.
I'll keep this issue open though to have it as a reminder for whenever that next major version will happen.

[gharlan]

isaacs/node-mkdirp#7

maybe you could update mkdirp from 0.5.1 to 0.5.3?

[rejas]

ah, true. ok, will do :-)