README: drop Ruby 1.8 support
glib2 README: drop Ruby 1.8 support
gtksourceview2 README: drop Ruby 1.8 support
gtksourceview3 README: drop Ruby 1.8 support
gvlc README: drop Ruby 1.8 support
gstreamer-no-gi README: drop Ruby 1.8 support
goocanvas README: drop Ruby 1.8 support
Merge pull request #168 from myokoym/readme-drop-ruby-1-8-support

Readme: drop Ruby 1.8 support

GitHub: fixes #166

Reported by Markus Heiler. Thanks!!!
Patch by Masafumi Yokoyama. Thanks!!!