MB-10496: Add jemalloc as a cbdeps dependancy for CentOS 6

This will cause jemalloc to be the preferred memory allocator for
CentOS 6.

Change-Id: I0335f87938d0917fff29c507b75cb382aee701dc
Reviewed-on: http://review.couchbase.org/44083
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>