Simplifiy `--debug` test
Use $VERBOSE to control warn output

Overriding Kernel#warn doesn't allow us to test warnings in
Gem::ConfigFile.  This was broken by @1bc83df because nobody remembers
that $VERBOSE controls Kernel#warn.
Print an extra message when --debug is set

Many people file bugs against RubyGems because they don't know what
--debug (and $DEBUG) do to ruby.  Now we print a message that will
hopefully make it a bit more clear.
Add #808 to History