fix: pull square.js from correct URL

src/templates/payment_form.html

@@ -5,7 +5,7 @@
 
 {% block extrahead %}
 
-<script type="text/javascript" src="web.squarecdn.com/v1/square.js"></script>
+<script type="text/javascript" src="https://web.squarecdn.com/v1/square.js"></script>
 {# bring the location IDs into javascript so the next bits know about them #}
 <script type="text/javascript">
 
@@ -215,4 +215,4 @@
     </div>
 </div>
 </div>
-{% endblock %}
+{% endblock %}
\ No newline at end of file