Use broccoli-less-single

The problem in #135 seems to be that the LESS and Stylus Broccoli plugins are fundamentally different from Sass plugin. The Sass plugin compiles a single, primary input file into a single output file, and takes a tree of dependencies (for imported files). The LESS plugin, on the other hand, just compiles each `.less` file into a `.css` file, one-to-one. It doesn't support imports, or multi-file dependencies.

I've just created a [new LESS plugin](https://github.com/gabrielgrant/broccoli-less-single), based on the Sass version's (imo) correct n-to-one semantics. This branch sets up ember-cli to use that plugin for (functioning!) LESS, when it's installed.

Fixes #136