Bump govuk_template_jinja to 0.19.0

https://github.com/alphagov/govuk_template/blob/master/CHANGELOG.md

- Remove external link styles (PR #231)
- Remove extraneous copy of HTML5shiv that wasn’t being used (PR #254)
- Update govuk_frontend_toolkit to latest version (PR #256)

package.json

@@ -16,7 +16,7 @@
     "express": "4.13.3",
     "express-nunjucks": "^0.9.3",
     "express-writer": "0.0.4",
-    "govuk_template_jinja": "0.18.3",
+    "govuk_template_jinja": "0.19.0",
     "grunt": "^0.4.2",
     "grunt-cli": "0.1.13",
     "grunt-concurrent": "^0.4.3",