[emacs] further refine my Emacs' unicode font stack

Ditch Hack and Symbola monospacified. Ubuntu Mono and DejaVu Sans Mono
are more than good enough. Also, force DejaVu Sans Mono for greek
alphabet, since IMHO they look better than Ubuntu Mono's. For example,
DejaVu's `α' looks much better and more distinguishable from `a' than
Ubuntu Mono's.