Improve build docs & dev setup.

[beporter]

Resolves #54.

Builds on #91 to add the existing asset pipeline tooling to (only) the vagrant box.

This work conflicts with #90 somewhat. In general, #90 is preferable as it moves things forward and this PR merely meets the project where it's at. If anything, I'd recommend merging this one first, then adapting #90 to:

1. Not use yarn. It's unnecessary these days.
2. Replace yarn.lock with package-lock.json.
3. Remove the unnecessary yui-compressor from the Vagrantfile provisioning block.