Replace quickTemp with broccoli-caching-writer.

This fixes #4 and should cache correctly when sass-tree is not changed.

I also fixed the test which was not passing for me. Test is passing again now.

I am using broccoli-merge-trees in this pr because this was the easiest way to not break backwards compatibility. For example ember-cli expects the first argument to the SassCompiler to be an array and most likely needs this behaviour to be able to swap out sass-compilers based on the sass broccoli plugin that is installed via npm.

Thanks for this project!