Add missing quotes to config.json template

This commit adds a missing pair of quotes around the config.json
template. The url isn't being treated as a string without the quotes,
and we're getting json parsing errors.

Change-Id: Ic6a435b3872de89801792296d19d98107e673e48

templates/config.json.erb

@@ -1,3 +1,3 @@
 {
-  "apiRoot": <%= @api_endpoint %>
+  "apiRoot": "<%= @api_endpoint %>"
 }