Upgrade to Rails 4.1.8

Due to security vulnerability, which doesn't affect this app, but it's
better to keep up with the latest stable releases, than lag behind.
Merge pull request #523 from ministryofjustice/rails-4.1.8

Upgrade to Rails 4.1.8
removed unneeded ruby- prefix from .ruby-version
updated govuk_frontend_toolkit; and added govuk_elements gem (not released yet)
replace old stylesheets with new govuk_elements_extensions sass file, which reuses govuk_elements sass
center content by wrapping content in main#page-container element; remove .page-header to fix header formatting
remove divider row between claimants
use govuk elements js to set border on focus for radio and checkbox labels
require bind.js to enable tests to run; phantomjs does not support bind() natively
add .js-enabled .hide and .js-only .js-enabled styles to fix show/hide functionality when JS enabled
change to form date css classes from govuk elements
import elements files separately as required; extend validation error styles
use .heading-small style for validation summary header
import elements/buttons