Integrate ruby-mode with electric-indent-mode better

Integrate ruby-mode with electric-indent-mode better

* lisp/progmodes/ruby-mode.el (ruby--at-indentation-p): New function,
extracted from `ruby-smie-rules'.
(ruby--electric-indent-chars): New variable.
(ruby--electric-indent-p): New function.
(ruby-mode): Use `electric-indent-functions' instead of
`electric-indent-chars'.