Update domains.rb
Update domains.rb

The domains:add had no function and would throw:

/Library/Ruby/Gems/2.0.0/gems/dokku-cli-0.1.1/lib/dokku_cli.rb:69:in `method_missing': undefined method `domains_add' for #<DokkuCli::Cli:0x007f9d84017998> (NoMethodError)
	from /Library/Ruby/Gems/2.0.0/gems/dokku-cli-0.1.1/lib/dokku_cli.rb:65:in `method_missing'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/command.rb:29:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/command.rb:126:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
	from /Library/Ruby/Gems/2.0.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /Library/Ruby/Gems/2.0.0/gems/dokku-cli-0.1.1/bin/dokku:5:in `<top (required)>'
	from /usr/bin/dokku:23:in `load'
	from /usr/bin/dokku:23:in `<main>'
vfulco/dokku-cli,SebastianSzturo/dokku-cli