Deprecate 'gdk run' in favor of 'gdk start'

We have had Runit in beta for a while now.

I think it's time to make it official.

Closes #624 (closed)

This will be felt by all GDK users! gdk run will stop working, and print a message instead:

% gdk run
'gdk run' is deprecated; use 'gdk start', 'gdk stop' and 'gdk tail' instead

If runit is not installed, the user will get an error with a installation hint (see !819 (merged)).

I have manually confirmed that gdk update works.

Because make pry was coupled to Foreman too much, I removed it and created gdk thin.