Install GOV.UK Template with NPM

Since https://github.com/alphagov/govuk_template/pull/193 the Jinja version of
the GOV.UK Template is published with a `package.json`. This means
- we can consume it via NPM
- so we can get rid of Bower

Which is what this commit does.

package.json

@@ -7,7 +7,6 @@
   },
   "scripts": {
     "test": "gulp lint",
-    "postinstall": "./node_modules/bower/bin/bower install",
     "build": "gulp",
     "watch": "gulp watch"
   },
@@ -21,9 +20,9 @@
   "dependencies": {
     "babel-core": "6.3.26",
     "babel-preset-es2015": "6.3.13",
-    "bower": "1.7.1",
     "govuk-elements-sass": "1.1.1",
     "govuk_frontend_toolkit": "4.6.0",
+    "govuk_template_jinja": "https://github.com/alphagov/govuk_template/releases/download/v0.16.4/jinja_govuk_template-0.16.4.tgz",
     "gulp": "3.9.0",
     "gulp-add-src": "0.2.0",
     "gulp-babel": "6.1.1",