www/gist: Unbreak runtime by update to 6.0.0

This port currently fails with a Ruby RuntimeError. Fix by update to
6.0.0. Also fix USE statement to depend on ruby. While here pacify
portlint, portclippy, portfmt.

PR: 271222, 271216
Approved by: portmgr (blanket, trivial fix)

(cherry picked from commit fb4a253201d411adbd18167a6ca0d6a8d9a55d1a)