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

@@ -22,7 +22,7 @@
     "express-writer": "0.0.4",
     "govuk-elements-sass": "2.0.0",
     "govuk_frontend_toolkit": "^5.0.0",
-    "govuk_template_jinja": "0.18.3",
+    "govuk_template_jinja": "0.19.0",
     "gulp": "^3.9.1",
     "gulp-clean": "^0.3.2",
     "gulp-nodemon": "^2.1.0",