converter_positions.yaml -> converter_positions.yml
Warn a user when converter positions are unsaved

If the user has edited converter positions, but forgot to save the YAML
file, they will receive a message reminding them to do so.

Closes #680
Remove unnecessary question

We don't need to ask the user's intent in the converter position
confirmation; the browser pop-up already asks it.
Update debugger to v1.6.6

Adds compatibility with Ruby 2.1.1.
Remove debug line from Slot
Update to Rails 4.0.3