Move d3 to package.json and include I18n JS library though Gemfile

Also, update bundle to make it installable again (a version had been removed)

postcss.config.js

@@ -0,0 +1,12 @@
+module.exports = {
+    plugins: [
+        require('postcss-import'),
+        require('postcss-flexbugs-fixes'),
+        require('postcss-preset-env')({
+            autoprefixer: {
+                flexbox: 'no-2009'
+            },
+            stage: 3
+        })
+    ]
+};