release_prep: Use puppetlabs_spec_helper 7.x

Author: bastelfreak

Gemfile

@@ -43,7 +43,7 @@ group :system_tests do
 end
 group :release_prep do
   gem "puppet-strings", '~> 4.0',         require: false
-  gem "puppetlabs_spec_helper", '~> 6.0', require: false
+  gem "puppetlabs_spec_helper", '~> 7.0', require: false
 end
 
 puppet_version = ENV['PUPPET_GEM_VERSION']