Fix wrong expected exception in spec assertion

It's pretty straightforward that an `ArgumentError` is raised here.
I wonder if this was changed to `RejectedExecutionError` because
somehow that was happening instead due to cross-spec interference.

Author: ivoanjo

spec/concurrent/channel_spec.rb

@@ -557,7 +557,7 @@ module Concurrent
       end
     end
 
-    context 'goroutines', notravis: true do
+    context 'goroutines' do
 
       let(:default_executor) { Channel.const_get(:GOROUTINES) }
 
@@ -595,7 +595,7 @@ module Concurrent
         it 'raises an exception when no block is given' do
           expect {
             Channel.go_loop
-          }.to raise_error(RejectedExecutionError)
+          }.to raise_error(ArgumentError)
         end
 
         it 'loops until the block returns false' do