Bump all dependencies that bring @ember/string < 4

Not sure if this is exhaustive or needed list, but those are my prime suspects now:

```
"ember-cli-string-helpers": "^6.1.0",
"ember-modal-dialog": "^4.1.4",
"ember-responsive": "^5.0.0",
"ember-classy-page-object": "0.8.0-beta.2",
```

This is to see if we can resolve the issue with `@ember/string` v3 once and for all.