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 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
Bumps Web Starter Kit to v0.5.1

Commits the generated output of `grunt wsk-version`.
update performance translation.
Fixes #958 - adds Russian
Fixes #960 - Adds cn-tw
Fixes #947 - Adds Arabic
Fixes #959 - Adds Turkish
Added comment about open file handles on Mac
Updates app engine handler for files with or without .html
Removing BOM from file
Add mention to Network Conditioning feature

Google Chrome has added a feature to its DevTools that throttles
the speed of a connection, emulating limited network conditions.

Fixes #964.
fix some sentences
More RTL support to fix navigation menus and footers
Fix bits per image channel calculation.
Fix landing pages to fall back to en
Restrict fallback to en
Have sub articles appear on landing
Fixes some odd quotes and other l10n artifacts.
Fixes meta-color and changes dist-&-disc to blue
Updates to betterbook to add distribution and discovery