Bump govuk_template_jinja to 0.18.2

https://raw.githubusercontent.com/alphagov/govuk_template/master/CHANGEL
OG.md

# 0.18.2

- Degrade gracefully when external JS can’t be loaded ([PR
#248](alphagov/govuk_template#248))

package.json

@@ -16,7 +16,7 @@
     "express": "4.13.3",
     "express-nunjucks": "^0.9.3",
     "express-writer": "0.0.4",
-    "govuk_template_jinja": "0.18.1",
+    "govuk_template_jinja": "0.18.2",
     "grunt": "^0.4.2",
     "grunt-cli": "0.1.13",
     "grunt-concurrent": "^0.4.3",