Update shelljs to 0.8.4

[friederbluemle]

0.8.4 includes shelljs/shelljs@05374a7 which fixes a bunch of warnings we keep getting when using Node.js 14:

(node:54809) Warning: Accessing non-existent property 'cat' of module exports inside circular dependency
(Use `node --trace-warnings ...` to show where the warning was created)
(node:54809) Warning: Accessing non-existent property 'cd' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'chmod' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'cp' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'dirs' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'pushd' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'popd' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'echo' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'tempdir' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'pwd' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'exec' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'ls' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'find' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'grep' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'head' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'ln' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'mkdir' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'rm' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'mv' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'sed' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'set' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'sort' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'tail' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'test' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'to' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'toEnd' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'touch' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'uniq' of module exports inside circular dependency
(node:54809) Warning: Accessing non-existent property 'which' of module exports inside circular dependency

There are not other changes compared to 0.8.3

More details:
shelljs/shelljs#991

Fixes #1558