Upgrade GOV.UK toolkit, template and elements

We should make sure that the parts of the UI that we inherit from these
packages are kept up to date.

This commit:
- updates each dependency to the latest version
- makes patch (bug fixing) version bumps automatic because
  - it makes less work for us
  - we don’t get so far behind
  - we should be able to trust dependencies that are coming from other
    teams in this building

Full changes that this brings in:

GOV.UK Template
---

- Increase skiplink colour contrast ([GOV.UK Template PR #263](https://github.com/alphagov/govuk_template/pull/263))

GOV.UK Elements
---
- Too many to show – see
  https://github.com/alphagov/govuk_elements/compare/v1.1.1...v3.0.2#diff-4ac32a78649ca5bdd8e0ba38b7006a1e

GOV.UK Frontend Toolkit
---
- Too many to show – see
  https://github.com/alphagov/govuk_frontend_toolkit/compare/v4.6.0...v5.2.0#diff-4ac32a78649ca5bdd8e0ba38b7006a1e

package.json

@@ -21,9 +21,9 @@
     "babel-core": "6.3.26",
     "babel-preset-es2015": "6.3.13",
     "diff-dom": "2.1.0",
-    "govuk-elements-sass": "1.1.1",
-    "govuk_frontend_toolkit": "4.6.0",
-    "govuk_template_jinja": "https://github.com/alphagov/govuk_template/releases/download/v0.19.1/jinja_govuk_template-0.19.1.tgz",
+    "govuk-elements-sass": "3.0.x",
+    "govuk_frontend_toolkit": "5.2.x",
+    "govuk_template_jinja": "0.19.x",
     "gulp": "3.9.0",
     "gulp-add-src": "0.2.0",
     "gulp-babel": "6.1.1",