[rubygems/rubygems] Use capture_output instead of capture_io for test-unit

https://github.com/rubygems/rubygems/commit/ab9c80d4cb

test/rubygems/test_deprecate.rb

@@ -141,7 +141,7 @@ class TestDeprecate < Gem::TestCase
   end
 
   def test_deprecated_method_outputs_a_warning_old_way
-    out, err = capture_io do
+    out, err = capture_output do
       thing = OtherThing.new
       thing.foo
       thing.foo_arg("msg")

test/rubygems/test_gem_package_task.rb

@@ -56,7 +56,7 @@ class TestGemPackageTask < Gem::TestCase
       g.summary = 'summary'
     end
 
-    _, err = capture_io do
+    _, err = capture_output do
       Rake.application = Rake::Application.new
 
       pkg = Gem::PackageTask.new(gem) do |p|