Replace pry with pry-debugger

From the [README](https://github.com/nixme/pry-debugger/blob/151ce464018de136ff0b97da77d259daef4f9c46/README.md):
> Adds step, next, finish, and continue commands and breakpoints to Pry using debugger.

I find said functionality essential for effective debugging and would like to have it at my disposal when debugging Stringer.