Update install templates to use jquery3

jQuery has known vulnerabilities in versions prior to `3.5.0`. This
commit updates the load configuration to require `jquery3` instead of
`jquery`. This will resolve the `jquery` version from `1.12.4` to
`3.5.1`, thereby alleviating the vulnerability for new installations.

backend/spec/javascripts/spec_helper.js

@@ -4,7 +4,7 @@
 //= require support/chai-jq-0.0.7
 
 //= require_self
-//= require jquery
+//= require jquery3
 //= require rails-ujs
 //= require spree/backend
 //= require_tree ./support