Replace ruby-debug19 with debugger for Ruby 1.9.x

I'm currently going through the Advanced Rails - Two Day tutorials and did the setup according to this:
Follow http://tutorials.jumpstartlab.com/topics/environment/advanced_tools.html to setup other specialty tools

The ruby-debug19 gem caused isssues when I started going through the 'Using the Debugger' tutorial. So for consistency I thought it might be better to setup initially with the debugger gem rather than ruby-debug19.