Import StripeJS mock from fake_stripe gem

This is the original from https://github.com/thoughtbot/fake_stripe/blob/v0.3.0/lib/fake_stripe/assets/v3.js

spec/support/fixtures/stripejs-mock.js

@@ -0,0 +1,34 @@
+class Element {
+  mount(el) {
+    if (typeof el === "string") {
+      el = document.querySelector(el);
+    }
+
+    el.innerHTML = `
+      <input id="stripe-cardnumber" placeholder="cardnumber" size="16" type="text">
+      <input placeholder="exp-date" size="6" type="text">
+      <input placeholder="cvc" size="3" type="text">
+      <input placeholder="postal" size="6" type="text">
+    `;
+  }
+}
+
+window.Stripe = () => {
+  const fetchLastFour = () => {
+    return document.getElementById("stripe-cardnumber").value.substr(-4, 4);
+  };
+
+  return {
+    elements: () => {
+      return {
+        create: (type, options) => new Element()
+      };
+    },
+
+    createToken: card => {
+      return new Promise(resolve => {
+        resolve({ token: { id: "tok_123", card: { last4: fetchLastFour() } } });
+      });
+    }
+  };
+};