Add centos builder.
Add centos omnibus builder.
Add enchant for sphinxcontrib-spelling.
Don't include commented find-links in ubuntu pip.conf.
Use elrepo kernel (for 3.17+).
Use same version of zfs on centos, as on fedora.

We need this for the new kernel.
Bump ZFS/SPL version on centos.
Add a note about needing to upgrade docker.