small fix for a node.js edge case

A Ractive instance's viewmodel should be 'applied' on initialisation, not just on render, so that `ractive.toHTML()` works correctly.

There's [one more test](https://github.com/ractivejs/ractive/blob/dev/test/samples/render.js#L447-L452) that's failing in node.js...
Inside `applyChanges()`, can we lazy check if computations need to be done? Then we can get rid of `viewmodel.compute()` above