Use version 4.18.0 of the govuk_frontend_toolkit

https://raw.githubusercontent.com/alphagov/govuk_frontend_toolkit/master
/CHANGELOG.md

# 4.18.0

- Add GOVUK.ShowHideContent JavaScript to support showing and hiding
content, toggled by radio buttons and checkboxes ([PR
alphagov#315](alphagov/govuk_frontend_toolkit#315)).

# 4.17.0

- SelectionButtons will add a class to the label with the type of the
child input ([PR
alphagov#317](alphagov/govuk_frontend_toolkit#317))

package.json

@@ -22,7 +22,7 @@
     "express-session": "^1.13.0",
     "express-writer": "0.0.4",
     "govuk-elements-sass": "1.2.0",
-    "govuk_frontend_toolkit": "^4.16.1",
+    "govuk_frontend_toolkit": "^4.18.0",
     "govuk_template_jinja": "0.18.1",
     "grunt": "0.4.5",
     "grunt-cli": "0.1.13",