Upgrade to shelljs 0.8.4

[iamareebjamal]

Node.js 14 is currently considering printing a warning when a non-existent
property of module.exports is accessed while in a circular require()
dependency, in order to make it easier to catch issues with circular
dependencies.

Refs: nodejs/node#29935

v0.8.4 included a commit to prevent circular dependency warnings in Node 14.

See also: shelljs/shelljs#973