Restore "Add pry and pry debuggers to common spree dependencies"

This reverts commit 7006953963f65e69c14c6f88f2a2e87e83d8b20d.

Was breaking CI but that should be fixable by updating bundler on the CI.

@huoxito Where was the CI error happening?  This should work fine even on Ruby 1.9.x as long as an up-to-date version of bundler is installed.
Example: https://gist.github.com/jordan-brough/05ad6d1387d23558b651