Use SHA1 for libevent

Change-Id: I973c78dddaf7b46bf9293a09fc1e5c476fb5e795
Reviewed-on: http://review.couchbase.org/28733
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>