Update handlebars dep to patch vulnerability

[jon-wei]

I saw the following message during a build:

[INFO] added 1153 packages from 1222 contributors and audited 91849 packages in 10.626s
[INFO] found 1 high severity vulnerability
[INFO]   run `npm audit fix` to fix them, or `npm audit` for details

npm audit reports the following:

$ npm audit
                                                                                
                       === npm audit security report ===                        
                                                                                
# Run  npm update handlebars --depth 5  to resolve 1 vulnerability
┌───────────────┬──────────────────────────────────────────────────────────────┐
│ High          │ Prototype Pollution                                          │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Package       │ handlebars                                                   │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Dependency of │ jest [dev]                                                   │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ Path          │ jest > jest-cli > istanbul-api > istanbul-reports >          │
│               │ handlebars                                                   │
├───────────────┼──────────────────────────────────────────────────────────────┤
│ More info     │ https://nodesecurity.io/advisories/755                       │
└───────────────┴──────────────────────────────────────────────────────────────┘
found 1 high severity vulnerability in 91849 scanned packages
  run `npm audit fix` to fix 1 of them.

This patch is the fix applied by npm audit fix.

[clintropolis]

LGTM 👍 at least this is only a dev dependency, so impact is probably minor if any, but still good to fix.