Update release-emacs.nix for nicer display in hydra.

- Don't display the -${system} prefix
- Map lowPrio over all legacy emacs packages

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Add default 'emacsPackages' attribute set, default to emacs24.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>
Add namePrefix to emacs packages.

Will default to emacs${majorVersion}-${package}.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>