Unnecessary package duplication

If you install `ember-cli-fastboot` 4.1.5, you will always get two different versions of `fastboot` in your dependency graph:

```
ember-cli-fastboot 4.1.5
├── fastboot 4.1.5
└─┬ fastboot-express-middleware 4.1.2
  └── fastboot 4.1.2
```

This is because `workspace:*` dependencies become exact version matches when published. 

It would probably be better to use `workspace:^`.