Using debugger gem

ruby-debug19 is not maintained actively anymore. Switching to debugger, so bundle gem installations are smooth.

Relevant links:

http://stackoverflow.com/questions/1083451/debugging-in-ruby-1-9/10414984#10414984
http://stackoverflow.com/questions/10474872/could-not-find-linecache19-0-5-13-in-any-of-the-sources-on-windows-7-x64
https://github.com/cldwalker/debugger