Pin gems to specific versions

https://github.com/alphagov/styleguides/blob/master/ruby.md#general
Remove foreman

This project does not contain a Procfile.
Bump Jekyll to 1.2.1
Switch to using jekyll-assets to include assets

This commit uses the jekyll-assets gem to generate nice asset paths
that are cachebusted properly.

Rename all the JavaScript and Stylesheets to put them in the _assets
directory. Put the toolkit submodule there too.

Remove the CSS check from the compile script. Compilation definitely
fails now if the Sass doesn't compile, as I discovered when it failed
because we have a stray image-url() in _typography.scss (removed).

Use the provided liquid tags to include the CSS and JS in the HTML
document.
Merge pull request #255 from alphagov/gems-gems-gems

Switch to using jekyll-assets for asset compilation