Updating OGL link text and destination.

Version 3 of the OGL was released on 31st October. This updates the link text and destination and adds the rel="license" attribute.
Bump govuk_frontend_toolkit to latest version

Changes include: new helpers for creating grid layouts, support for selection-button events to be document-level, new grid helpers using %site-width-container for IE. Full details in the changelog - https://github.com/alphagov/govuk_frontend_toolkit/blob/master/CHANGELOG.md.
Merge pull request #100 from alphagov/update_ogl_licence

Update OGL licence links in footer, bumping frontend_toolkit