Add locking feature to prevent stale code

Adds locking feature that works with [ember-cli-rails-addon](https://github.com/rondale-sc/ember-cli-rails-addon) (the companion addon in node land)

The `ember-cli-rails-addon` uses the `preBuild` hook to write a file called `preBuild.lock` to the output and the `postBuild` hook to delete that file and write `postBuild.lock`

The middleware we are now injecting blocks until the `preBuild` file is not found and the `postBuild` **is** found before continuing the request.  This results in a block until ember-cli has fully built the output.

Cheers :beer:
I am unsure that you should care too much about this here.  You can make the addon check the required version `var version = this.project.emberCLIVersion();` and it can be kept with a nice message.  Then here (in ember-cli-rails) you just don't have to care about it.