(maint) Bump PDK to 2

PDK 2 has been released which now accepts Facter 4, so we can use it
now.

Author: GabrielNagy

Gemfile

@@ -51,7 +51,7 @@ end
 
 group :release do
   gem "puppet-blacksmith", '~> 3.4',                                             require: false
-  gem "pdk",                                                                     platforms: [:ruby]
+  gem "pdk", *location_for(ENV['PDK_VERSION'] || '~> 2')
 end
 
 puppet_version = ENV['PUPPET_GEM_VERSION']