Tie our AUFS checkout to an explicit commit to fix repeatability better

This commit comes from where AUFS was at the previous build of the
boot2docker ISO, which is known to be working (where latest HEAD from
the upstream aufs3.16 branch doesn't currently compile properly).
Merge pull request #658 from tianon/aufs-determinism

Tie our AUFS checkout to an explicit commit to fix repeatability better