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

This reverts commit 7c367d5ca0e2f293813278a0e37d9f731aa8befb.

CI breaks with these errors: (lack of ruby 2.1 apparently)

  [03:52:55][Step 1/1] in directory: /mnt/72aec43634788be0/core
  [03:52:55][Step 1/1] `ruby_21` is not a valid platform. The available options are: [:ruby, :ruby_18,
  [03:52:55][Step 1/1] :ruby_19, :ruby_20, :mri, :mri_18, :mri_19, :mri_20, :rbx, :jruby, :mswin,
  [03:52:55][Step 1/1] :mingw, :mingw_18, :mingw_19, :mingw_20]
  [03:52:56][Step 1/1] `ruby_21` is not a valid platform. The available options are: [:ruby, :ruby_18,
  [03:52:56][Step 1/1] :ruby_19, :ruby_20, :mri, :mri_18, :mri_19, :mri_20, :rbx, :jruby, :mswin,
  [03:52:56][Step 1/1] :mingw, :mingw_18, :mingw_19, :mingw_20]
  [03:52:56][Step 1/1] `ruby_21` is not a valid platform. The available options are: [:ruby, :ruby_18,
  [03:52:56][Step 1/1] :ruby_19, :ruby_20, :mri, :mri_18, :mri_19, :mri_20, :rbx, :jruby, :mswin,
  [03:52:56][Step 1/1] :mingw, :mingw_18, :mingw_19, :mingw_20]