make a sample script gtk3 ready

gtk3/sample/misc/assistant.rb
* add "round" method call for "progress.fraction" value not to exceed 1.0.
* remove the last "full_featured_assistant" sample because Gtk::Assistant#set_page_side_image and Gtk::Assistant#set_page_header_image methods have been deprecated and there is no alternative method.
Merge pull request #244 from kitachro/assistantrb

Make one sample script gtk3 ready

Patch by kitachro. Thanks!!!