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

[scpeters]

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The brew ruby command was broken by #3851 (cannot load such file -- cmd/irb (LoadError)):

$ brew ruby -e 'exit 0'
/usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- cmd/irb (LoadError)
	from /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.3/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'

The irb command was moved to dev-cmd but we forgot to update the path embedded in the ruby command. This fixes the path and adds a very basic test for the ruby command.

[MikeMcQuaid]

Oops, thanks for the fix!