Update shibboleth2.xml.erb

To fix warning in shibd_warn.log:

---8<---
2013-06-18 13:19:37 WARN Shibboleth.Application : insecure cookieProps setting, set to "https" for SSL/TLS-only usage
2013-06-18 13:20:46 WARN Shibboleth.Application : insecure cookieProps setting, set to "https" for SSL/TLS-only usage
---8<---

AASC should be secure by default and use HTTPS so setting:

      <Sessions lifetime="28800" timeout="3600" relayState="ss:mem"
                  checkAddress="false" handlerSSL="true" cookieProps="https">

rather than 

http