Fix warning from ember-decorators

Since #113, the following warning appears when building or testing:

```
WARNING: ember-decorators (used in ember-popper): You have not installed @ember-decorators/babel-transforms. It has been extracted to a separate addon. See instructions for installation: https://github.com/ember-decorators/babel-transforms#readme
```

I tried to install ember-decorators/babel-transforms as described in the linked readme, but it fails with

```
$ ember install @ember-decorators/babel-transforms
NPM: Installed @ember-decorators/babel-transforms
Install failed. Could not find addon with name: @ember-decorators/babel-transforms
```

It does add an entry to package.json and is listed as a `devDependency`, but running a build or a test always yields that error from ember-popper.

Addressing this warning may require ember-popper to fix upstream

Reproduce the warning during a build or test and inspect ember-popper's handling of ember-decorators. Compare the package.json devDependency entry with the installation instructions in the linked @ember-decorators/babel-transforms README. Done means the warning is resolved, or the required upstream change is clearly identified.

Written by the indexing model from the issue text.