Update node-webkit to 0.10.0

`link` stanza has been removed as it does not serve normal usage of
node-webkit.

node-webkit is a runtime targeted by application developers.
Usage of node-webkit usually takes one of two forms:
- launching the binary with development code as argument, either by
  CLI or drag-and-drop.
- bundling a copy of the binary with release code, for packaging and
  distribution.

Neither usage is aided by a symlink; a known binary location, which
Cask provides, is sufficient.

Addendum: `version` value is reused in `url` stanza to facilitate
future updates.
Merge pull request #5470 from ndr-qef/node-webkit

Update node-webkit to 0.10.0 and remove `link` stanza