Update braintree dropin version.

templates/accounts/billing.html

@@ -514,7 +514,7 @@
 {% endblock %}
 
 {% block scripts %}
-<script src="https://js.braintreegateway.com/web/dropin/1.8.0/js/dropin.min.js"></script>
+<script src="https://js.braintreegateway.com/web/dropin/1.17.1/js/dropin.min.js"></script>
 {% compress js %}
 <script src="{% static 'js/jquery-2.1.4.min.js' %}"></script>
 <script src="{% static 'js/bootstrap.min.js' %}"></script>