Install ruby-dev package in default recipe to provide mkmf in building n...

...ative extensions.

This will prevent the ruby build failing with:

08:36:25: /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
08:36:25: : cannot load such file -- mkmf (LoadError)
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from extconf.rb:17:in `<main>'