Missed Gemfile change in f849cfe3

Gemfile

@@ -5,10 +5,6 @@ gemspec
 # We need a newish Rake since Active Job sets its test tasks' descriptions.
 gem 'rake', '>= 10.3'
 
-# Active Support depends on a prerelease concurrent-ruby 1.0.0, so track
-# latest master as it approaches release.
-gem 'concurrent-ruby', '~> 1.0.0.pre3', github: 'ruby-concurrency/concurrent-ruby'
-
 # Active Job depends on the URI::GID::MissingModelIDError, which isn't released yet.
 gem 'globalid', github: 'rails/globalid', branch: 'master'
 gem 'rack', github: 'rack/rack', branch: 'master'