Limit concurrent htmlmin to dist (fixes #14)

It looks like we want to limit to `htmlmin:dist` in the `concurrent:dist` task.

As it is now, in the build step, `htmlmin:deploy` is run when we call `concurrent:dist`.  If `htmlmin:deploy` is run before `usemin`, it won't replace all the scripts in index.html with the combined, minified versions. And we get 404s :(