Automatically update the Web Starter Kit latest release download URL

`Grunt build` will trigger the `wsk-version` task, which in turn uses
[`latest-release`](https://www.npmjs.org/package/latest-release) to
determine the name, tag and zip-archive download URL of the latest
release of [google/web-starter-kit](https://github.com/google/web-starter-kit/releases/latest) on Github.

The results of this are written to `src/site/_config-wsk-version.yml`.

`Jekyll.destination.config` has been expanded to pick this config .yml up
load in the variables when building the site.

src/site/_en/starter-kit/index.html uses the {{site.wsk-zip-url}}
variable to point it's download button the to latest version.

See discussion in:
- google/web-starter-kit#414
- google/WebFundamentals#547

// @addyosmani