Fix just-extend vulnerability

[jdjkelly]

• https://www.npmjs.com/advisories/780
• Fixed via npm audit fix

npm audit results gave me this:

┌───────────────┬──────────────────────────────────────────────────────────────┐
│ Moderate      │ Prototype Pollution                                          │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Package       │ just-extend                                                  │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Dependency of │ yeoman-test [dev]                                            │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Path          │ yeoman-test > sinon > nise > just-extend                     │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ More info     │ https://npmjs.com/advisories/780                             │
└───────────────┴──────────────────────────────────────────────────────────────┘

Used npm version 6.8.0 to run the npm audit fix command to automatically resolve - it added additional optional: true parameters to several dependencies - this may be related to npm/npm#17722

[zone117x]

Latest dev branch currently passes npm audit