Change ruby-backward-sexp to backward-word on ruby-electric-space

In this version of ruby-electrict.el, `ruby-electric-space` fails with emacs snapshot (v2.3.50.1)

The error message is the following:
````
smie-forward-sexp-command: Scan error: "Containing expression ends prematurely", 581, 581
````

I tracked down the error to be on the execution of `ruby-backward-sexp`.
I have changed `ruby-backward-sexp` to `backward-word` on ruby so that the behavior is normal again, although you do have expansions with symbols (:def, :do, etc.), which I personally think is not a major issue.