Update govuk template mustache to the latest version

[gemmaleigh]

https://github.com/raw/alphagov/govuk_template/master/CHANGEL
OG.md
#0.18.1

• Remove gov.uk prefix from relative links when printing (PR
  #234)
• Fix a .visually-hidden bug on GOV.UK (PR
  #177)
  #0.18.0
• Publish the Jinja version of the template to NPM
• Update HTML5 Shiv to the latest version
• Remove an errant font loader script that was only being used for IE8
  #0.17.3
• Fix colour of H2 headings in footer
  #0.17.2
• Fix a bug with the skip-to-content link and iOS Voiceover
• Migrate @Viewport statement from govuk_frontend_toolkit
  #0.17.1
• Reduce file size of template: removes HTML comments, type
  attributes on
  scripts, and uses HTML5 charset declaration. #208
• Switch external link media query to be mobile first #205
• Sass file cleanups
  • Replace old grid mixins with newer grid from frontend toolkit #134
  • Remove duplicate grey variables #201
    #0.17.0
• Add CSS hook (.js-hidden) for hiding content when JS is enabled.
  Some apps
  have an equivalent hook, which can be removed once upgraded to this
  version
  #0.16.4
• Fix publish the Jinja version of the template with a package.json
  for those
  consuming it with NPM
  #0.16.3
• make the Django version of the template into a proper Python package
• publish the Jinja version of the template with a package.json for
  those
  consuming it with NPM
  #0.16.2
• more static assets added to assets.precompile to improve
  compatibility with apps running rails > 4.2.5
  #0.16.1
• Fix colour of logo when in :active state