Don't pass hardcoded input path to preprocessCss.

[This line](https://github.com/stefanpenner/ember-cli/blob/master/lib/preprocessors.js#L74) in preprocessors.js doesn't work if you're using a plugin with multiple extensions (e.g., broccoli-ruby-sass) and a passing a custom `trees.styles` path to EmberApp.

The hardcoded string `'/app/styles'` is passed to preprocessCss as `inputPath` by EmberApp#styles, but that path might not actually exist on the filesystem.