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`.
Automatically update the WSK URL

Using the `wsk-version` task, which in turn uses the NPM package
`latest-release`, the `jekyll:appengine` task now automatically
updates the WSK URL to point to the latest release.

Fixes #708.

Authored-by: jbnicolai
Committed-by: jacquerie
Consolidate clean and canonical_url

Create a new filter called `canonicalize`, and replace every
instance of `clean` and `canonical_url` with it.

Fixes #984.
Add language tag to issue reporter

Prepend a tag such as '[EN]' to an issue, so that it's easier to
determine which language is the user referring to when she gives
feedback.

Fixes #1001.
Merge pull request #1002 from jacquerie/issue-1001

Add language tag to issue reporter
Fix few typos.
M40 updates: minlength and reportValidity().
Merge pull request #1003 from nbeloglazov/master

Fix two typos.
Merge pull request #988 from jacquerie/issue-984

Consolidate clean and canonical_url
Merge pull request #995 from jacquerie/issue-708

Automatically update the WSK URL
Make sure the build does npm install
Merge pull request #1007 from jpmedley/master

Chrome M40 updates: minlength and reportValidity().
M39 Update: meta them-color.
Merge pull request #1008 from jpmedley/master

M39 Update: meta them-color.
Merge pull request #1012 from jacquerie/master

Remove ruby reference from introduction