travis: don't require input
travis: use PPA only if it is needed
travis: install GStreamer good plugins
poppler test: remove test_render

`test_render` is severe environment dependency.
As reported #234, this test passed in Debian sid environment, but it
got failure in Ubuntu Trusty(14.04 LTS) environment.
So, it assumes that we cannot maintain this test. It should remove it.
gst: add Caps.any and Caps.empty
gst: support property for Gst::ChildProxy

GitHub: fix #233

Reported by saepia. Thanks!!!
gtksourceview3: suppress previous definition warning

`markup` and `text` are property.
gtksourceview3-gi: add experimental GObjectIntrospection based Ruby/GtkSourceView3 bindings
gtk3: add Gtk::Window#set_titlebar (enable the use of Gtk::HeaderBar)
Merge pull request #236 from tescorg/master

gtk3: add Gtk::Window#set_titlebar (enable the use of Gtk::HeaderBar)

Patch by tescorg. Thanks!!!
travis: use the latest Ruby 2.1
poppler: install Poppler with --with-glib option for Homebrew

GitHub: fix #237

Reported by Colin Dean. Thanks!!!
clutter-gstreamer: remove needless note
clutter-gstreamer: show text on control

Because a segmentation fault has been fixed.
clutter-gstreamer: call Gst.init
clutter-gstreamer: remove needless note
clutter-gstreamer: remove needless note
Add Ruby 2.1 because it is already supported
gtk3: use spaces for indent

    % inplace "sed -e 's/\t/        /g'" *.rb
gtk3: remove trailing spaces

    % inplace "sed -e 's/ *$//g'" *.rb