Use version 4.16.1 of the govuk_frontend_toolkit

# 4.16.1

- Fix anchor-buttons.js to trigger a native click event, also rename to
shimLinksWithButtonRole.js to explain what it does
- Add tests for shimLinksWithButtonRole ([PR
alphagov#310](alphagov/govuk_frontend_toolkit#310))

# 4.16.0

- Add Department for International Trade organisation ([PR
alphagov#308](alphagov/govuk_frontend_toolkit#308))

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": "^4.15.0",
+    "govuk_frontend_toolkit": "^4.16.1",
     "govuk_template_jinja": "0.18.1",
     "grunt": "0.4.5",
     "grunt-cli": "0.1.13",