Put the memory manager last, if it is found.

This means that it is more likely that we will pick a plugin that is actually
relevant to real-world usage.

Change-Id: I658c7b3968aa131fc1cd296f53fd202f9f1a6682
Reviewed-by: Renato Araujo Oliveira Filho <renato.filho@canonical.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>