Fix issue with send_messages in alerts utils

Wasn't opening the connection first (according to Django's docs).  Not
100% this works - we'll see in the sandbox.

oscar/apps/customer/alerts/utils.py

     # Send all emails in one go to prevent multiple SMTP
     # connections to be opened
     connection = mail.get_connection()
+    connection.open()
     connection.send_messages(emails)
     connection.close()