ruby-mode: Align def...end to the beginning of statement

ruby-mode: Align def...end to the beginning of statement

* lisp/progmodes/ruby-mode.el (ruby-alignable-keywords): New constant.
(ruby-align-to-stmt-keywords): Change the default value.  Use
`ruby-alignable-keywords' to generate the possible customization
choices.
(ruby-smie-rules): Instead of using a hardcoded list of alignable
keywords, check against the value of `ruby-alignable-keywords'
(http://lists.gnu.org/archive/html/emacs-devel/2014-01/msg01439.html).