Make sure ember-cli-dependency-checker is present

[rwz]

Without it there could be a situation when one developer runs npm install --save-dev ember-cli-rails-addon and commits changes to package.json, but then other developer pulls the change and runs rails without running npm install.

NPM doesn't actually perform any dependency checking before executing the code, so builds are running without the addon, and all sorts of weird shit happening. :(

[rwjblue]

ember-cli-rails-addon can throw or console.error if it is not present in package.json.

[rwz]

The problem is the gem only checks package.json, but not the actual presence of stuff in node_modules. We had an issue when package.json check passed successfully and builds were happening, but the addon was not installed and loaded.

So, when addon is not running, adding checks to it won't solve the problem.

I think the other thing we could/should do is to make addon check smarter and actually make it go deeper into node-modules to eliminate these situations.

[rwjblue]

Submitted #36 and rondale-sc/ember-cli-rails-addon#4 to assist in this.