title: Unicorn with Ruby 2.4.1 causing weird crash
body: <p>I am upgrading from Ruby 2.3.1 to Ruby 2.4.1 and upon doing so Unicorn seems be incompatible with the new version. I get the errors below. I am using Unicorn 5.1.0 and have tried Unicorn 5.3.1 to no avail. Do I need to compile with a different lib instead of XCode Tools?</p>
  
  <p>I get the error immediately upon starting the server with <code>foreman start</code> and the Procfile:</p>
  
  <pre><code>webpack: bin/webpack-dev-server
  gulp: gulp
  redis: ./scripts/start_redis_server.sh
  sidekiq: bundle exec sidekiq -C config/sidekiq.yml
  annotations_server: ./scripts/start_annotation_server.sh
  rails: bundle exec unicorn_rails -p 3000 -c config/unicorn.rb
  </code></pre>
  
  <p>It seems to start and run fine if I use a simple <code>bin/rails s</code> which I why I think it is specific to Unicorn.</p>
  
  <pre><code>objc[40847]: +[__NSPlaceholderDictionary initialize] may have been in progress in another thread when fork() was called.
  objc[40846]: +[__NSPlaceholderDictionary initialize] may have been in progress in another thread when fork() was called.
  objc[40846]: +[__NSPlaceholderDictionary initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
  objc[40847]: +[__NSPlaceholderDictionary initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
  E, [2017-10-05T12:12:01.618013 #40833] ERROR -- : reaped #&lt;Process::Status: pid 40847 SIGABRT (signal 6)&gt; worker=1
  E, [2017-10-05T12:12:01.618064 #40833] ERROR -- : reaped #&lt;Process::Status: pid 40846 SIGABRT (signal 6)&gt; worker=0
  I, [2017-10-05T12:12:01.619387 #40850]  INFO -- : Refreshing Gem list
  I, [2017-10-05T12:12:01.620867 #40851]  INFO -- : Refreshing Gem list
  </code></pre>