Avoid crash if GBPUSD comes back as None.

templates/index.html

@@ -101,7 +101,9 @@
 
     <ul>
     <li>Today is {{now.strftime("%A, %-d %b %Y")}}</li>
-    <li>GBPUSD: {{"{:,.3f}".format(gbpusd)}}</li>
+    {% if gbpusd %}
+      <li>GBPUSD: {{"{:,.3f}".format(gbpusd)}}</li>
+    {% endif %}
     <li>GWR advance ticket furthest date:
       {% if gwr_advance_tickets %}
         {{ gwr_advance_tickets.strftime("%A, %-d %b %Y") }}