`$SAFE = 4;` has been removed with Ruby 2.1

For background -
https://bugs.ruby-lang.org/issues/8468
Changset - https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/41259/diff/test/ruby/test_thread.rb
Merge pull request #12530 from vipulnsward/remove_thread_safe

`$SAFE = 4;` has been removed with Ruby 2.1