Switch to vtree dependency.

VirtualNode and VirtualText were moved to the `vtree` module.
If we're following semver, until vtree is on a stable API (1.x.y) any release can introduce breaking API changes.

Let's say `vtree@0.0.x` is targeted... a new release that breaks the package but satisfies semver could be installed (same behavior as with `*`).

If a specific version is targeted, such as `0.0.4`, you might end up with multiple versions of compatible `vtree` in your JS bundle.

Until a stable API is released, we either run the risk of multiple versions in client JS or installing a breaking version no matter how we specify the dependency version.

I suggest having all virtual-dom modules peer depend on `0.0.x`, with the tacit assumption that the API will not change until `0.1.0`.