dev-cmd/ruby: fix path to dev-cmd/irb and add test

Library/Homebrew/dev-cmd/ruby.rb

@@ -8,6 +8,6 @@ module Homebrew
   module_function
 
   def ruby
-    exec ENV["HOMEBREW_RUBY_PATH"], "-I#{HOMEBREW_LIBRARY_PATH}", "-rglobal", "-rcmd/irb", *ARGV
+    exec ENV["HOMEBREW_RUBY_PATH"], "-I#{HOMEBREW_LIBRARY_PATH}", "-rglobal", "-rdev-cmd/irb", *ARGV
   end
 end

Library/Homebrew/test/dev-cmd/ruby_spec.rb

@@ -0,0 +1,13 @@
+describe "brew ruby", :integration_test do
+  it "executes ruby code with Homebrew's libraries loaded" do
+    expect { brew "ruby", "-e", "exit 0" }
+      .to be_a_success
+      .and not_to_output.to_stdout
+      .and not_to_output.to_stderr
+
+    expect { brew "ruby", "-e", "exit 1" }
+      .to be_a_failure
+      .and not_to_output.to_stdout
+      .and not_to_output.to_stderr
+  end
+end