Bring back ruby-debug19 for Ruby versions < 1.9.3

Gemfile

@@ -25,7 +25,8 @@ platforms :mri_18 do
 end
 
 platforms :mri_19 do
-  gem "ruby-debug19"
+  # TODO: Remove the conditional when ruby-debug19 supports Ruby >= 1.9.3
+  gem "ruby-debug19" if RUBY_VERSION < "1.9.3"
 end
 
 platforms :ruby do