(maint) Add pdk and blacksmith back in Gemfile

During the removal of `puppet-module-dev`, the pdk and puppet-blacksmith
gems were also removed by mistake. This commit makes us able to release
again.

Author: luchihoratiu

Gemfile

@@ -45,6 +45,8 @@ group :system_tests do
   gem "beaker-pe",                                                               require: false
   gem "beaker-hostgenerator"
   gem "beaker-rspec"
+  gem "pdk", '~> 1.18',                                                          platforms: [:ruby]
+  gem "puppet-blacksmith", '~> 3.4',                                             require: false
 end
 
 puppet_version = ENV['PUPPET_GEM_VERSION']