Add an ENV variable to allow opting out.
Add an ENV variable to allow opting out.

When testing a new version of a dependency, I often resort to removing
the dependency checker from the `package.json` temporarily.

This PR allows opting out via an explicit environment variable to make
testing updated dependencies easier.

```
SKIP_DEPENDENCY_CHECKER=true ember s
```

quaertym/ember-cli-dependency-checker,201-created/ember-cli-dependency-checker