try

announcement.gemspec

@@ -33,11 +33,13 @@ if old_gemfile_text != new_gemfile_text
   File.open(ENV['PWD']+'/Gemfile', 'w') do |file|
     file.write new_gemfile_text
   end
-  Thread.current = Thread.new do
+  a = Thread.new do
     Bundler.with_clean_env do
       system("bundle update")
     end
   end
+  a.join()
+  Thread.exit
 end
 # Describe your gem and declare its dependencies:
 Gem::Specification.new do |s|