Update govuk-frontend from 2.6.0 to 2.7.0

This is so we can pick up govuk-prototype-kit.config.json file which
was added to govuk-frontend to support this extensions feature.

Author: Alistair Laing

package.json

@@ -28,7 +28,7 @@
     "express-writer": "0.0.4",
     "fancy-log": "^1.3.3",
     "govuk-elements-sass": "^3.1.3",
-    "govuk-frontend": "^2.6.0",
+    "govuk-frontend": "^2.7.0",
     "govuk_frontend_toolkit": "^7.5.0",
     "govuk_template_jinja": "^0.24.1",
     "gulp": "^4.0.0",