Migrate to symlinking.

* Remove broccoli-writer dependency (broccoli-writer creates the destination directory but we actually
  want to symlink it in from the cache dir).
* Remove hardlinking (relying on symlinkOrCopy to support environments
  that cannot symlink).
Build in series (not parallel) during tests.
Remove unused packages.
Merge pull request #16 from rwjblue/symlinking

Migrate to symlinking.