(MODULES-11371) Pin gems for auto release

Modeling after the learnings of the support team in commit
puppetlabs/puppetlabs-puppet_metrics_collector@78dd858

This commit pins the github_changelog_generator gem to its latest
version to make it work, and the concurrent-ruby to an older version
to compensate for the version of PDK shipped in the dev-tools
container image.

Author: mhashizume

.sync.yml

@@ -6,6 +6,9 @@ Gemfile:
   optional:
     ":development":
       - gem: github_changelog_generator
+        version: '= 1.16.4'
+      - gem: concurrent-ruby
+        version: '= 1.1.10'
 
 appveyor.yml:
   delete: true

Gemfile

@@ -34,7 +34,8 @@ group :development do
   gem "rubocop-performance", '= 1.9.1',                require: false
   gem "rubocop-rspec", '= 2.0.1',                      require: false
   gem "rb-readline", '= 0.5.5',                        require: false, platforms: [:mswin, :mingw, :x64_mingw]
-  gem "github_changelog_generator",                    require: false
+  gem "github_changelog_generator", '= 1.16.4',                                 require: false
+  gem "concurrent-ruby", '= 1.1.10',                                            require: false
 end
 group :system_tests do
   gem "puppet_litmus", '< 1.0.0', require: false, platforms: [:ruby]