Adds ruby-define-method:new_instance_method command converts a line of text into a method name with shift+enter like in Textmate. Please note: I'm still learning my way around Atom, improvements are welcomed!
To change the keybinding:
'.editor[data-grammar~=ruby]': 'shift-enter': 'ruby-define-method:new_instance_method'
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.