To make sure latest version is used during build

--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -33,6 +33,7 @@
   end
 end
 
+$:.unshift File.expand_path('../../lib/concurrent-ruby', __FILE__)
 require 'concurrent'
 $:.unshift File.expand_path('../../lib/concurrent-ruby-edge', __FILE__)
 require 'concurrent-edge'