travis: fix unable to fetch error

following error occured in Travis CI:
Failed to fetch
http://us.archive.ubuntu.com/ubuntu/pool/main/g/gtk+3.0/gir1.2-gtk-3.0_3.4.2-0ubuntu0.5_amd64.deb
404  Not Found
gtksourceview2: fix GtkSource#get_mark_category_background return value

It should return nil when mark_category_background isn't set. The
document of the method also says it. So this is an implementation bug.

GitHub: fix #215

Reported by mtasaka. Thanks!!!
gi test: maybe fix test failure on i686

GitHub: fix #216

Reported by mtasaka. Thanks!!!
gi test: follow (maybe) GObjectIntrospection change

GitHub: #217

Reported by mtasaka. Thanks!!!
gi test: ensure using "startup" signal
gi test: follow (maybe) GObjectIntrospection change

GitHub: fix #217

Reported by mtasaka. Thanks!!!
gdk3: fix a typo

GDK_TYPE_DISPLAY_MANAGER ->
GDK_TYPE_DEVICE _MANAGER
         ^^^^^^^
gdk3: add Gdk::Display#device_manager
gdk3: add Gdk::Window#display
clutter-gtk: add a check for duplicate Clutter.init

Because the following error occurred:

    % ruby -r clutter-gtk -e "Clutter::Color; ClutterGtk::Actor"
    /.../clutter-gtk-2.1.0/lib/clutter-gtk.rb:44:in `init': undefined method `init' for Clutter:Module (NoMethodError)
            from /.../clutter-gtk-2.1.0/lib/clutter-gtk.rb:33:in `const_missing'
            from -e:1:in `<main>'