fix(tests): use aufs in builder functional tests

We had previously switched to Ubuntu's default for Docker,
devicemapper. But aufs has proven to be faster and more reliable
for the CI system itself, and when we switched CI nodes to aufs we
did not do so with the builder functional test itself.
Merge pull request #1443 from deis/aufs-in-builder-tests

fix(tests): use aufs in builder functional tests