[build] bundle update ruby when versions get changed so lockfile is correct

Author: titusfortner

Rakefile

@@ -701,6 +701,7 @@ namespace :rb do
     File.open(file, "w") { |f| f.puts text }
 
     Rake::Task['rb:changelog'].invoke unless new_version.include?('nightly')
+    sh 'cd rb && bundle update'
   end
 end