fix(Makefile): ensure the installer makes /var/lib/deis/units readable

This only happens when running our installer via `sudo` (as we recommend in the one-liner install, unfortunately). Doing "sudo deisctl refresh-units" creates the dirs as 0755 and the files as 0644 properly. I'm unclear as to why the permissions are applied differently when run through the `makeself` installer, but adding on the `chmod` commands to the installer's final script does fix it for all cases I could test.
I'd prefer an explicit `chmod 755 /var/lib/deis /var/lib/deis/units`.