DEV: Remove deprecations & improve error handling (#12)

- Replace deprecated methods on client
- Fix broken dropdowns due to select kit 2 upgrade
- Graceful error handling when Stripe keys are not configured but plugin enabled

test/javascripts/helpers/stripe.js.es6

@@ -1,8 +1,10 @@
+import { Promise } from "rsvp";
+
 export function stubStripe() {
   window.Stripe = () => {
     return {
       createPaymentMethod() {
-        return new Ember.RSVP.Promise(resolve => {
+        return new Promise(resolve => {
           resolve({});
         });
       },