Update ShellJS for compatibility with Node.js 14

Resolves warnings (shelljs/shelljs#991):

    $ node --version
    v14.1.0
    $ npm run test

    > git-rev-sync@2.0.0 test /home/ak/src/git-rev-sync-js
    > node tests/index

    tests passed
    (node:143612) Warning: Accessing non-existent property 'cat' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'cd' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'chmod' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'cp' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'dirs' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'pushd' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'popd' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'echo' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'tempdir' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'pwd' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'exec' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'ls' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'find' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'grep' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'head' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'ln' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'mkdir' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'rm' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'mv' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'sed' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'set' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'sort' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'tail' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'test' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'to' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'toEnd' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'touch' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'uniq' of module exports inside circular dependency
    (node:143612) Warning: Accessing non-existent property 'which' of module exports inside circular dependency

package.json

@@ -67,7 +67,7 @@
   "dependencies": {
     "escape-string-regexp": "1.0.5",
     "graceful-fs": "4.1.15",
-    "shelljs": "0.7.7"
+    "shelljs": "0.8.4"
   },
   "devDependencies": {}
 }