MB-10374: Update snappy version to commit that fixes the recent CentOS 5 build failure. Correct external-override manifests to use only external git remotes.

Change-Id: Iae988ed33961d504a5730454efa10d15ee22b686
Reviewed-on: http://review.couchbase.org/34230
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>