Add enh-ruby-mode autoload; override ruby-mode

Add an autoload tag for enh-ruby-mode so it's available to call without
a require when installed from MELPA.

Add autoloaded auto-mode-alist settings for *.rb, Rakefile and *.gemspec
files to use enh-ruby-mode automatically. This overrides the normal
ruby-mode association so these files use enh-ruby-mode automatically.