Run this from the command line:
$ gem install ruby-debug19 -- --with-ruby-include=/Volumes/HoneyBadger1TB/Users/lex/.rbenv/versions/1.9.3-p194/include/ruby-1.9.1/ruby-1.9.3-p194
Fetching: ruby-debug-base19-0.11.25.gem (100%)
Building native extensions. This could take a while...
Fetching: ruby-debug19-0.11.6.gem (100%)
Successfully installed ruby-debug-base19-0.11.25
Successfully installed ruby-debug19-0.11.6
2 gems installed
Note that your path will vary. What's important is that you specify the directory of your Ruby header files with the with-ruby-include setting
Include the following in your Gemfile:
gem 'ruby-debug19', :require => 'ruby-debug'