Remove all traces of ruby-debug19 or debugger

Originally, there was a comment in the Gemfile that said to install ruby-debug19. That fell out maintenance, so that comment was wrong. Since programmers would most likely be using a debugger at some point anyway, I removed the comments directing developers to install any debugger gem. Instead, let the developer decide whether to use a debugger at all.

Fixes #132

Signed-off-by: Michaux Kelley <michaux.kelley@scimedsolutions.com>