travis: use 2.1.1 instead of 2.1.0

I removed 2.1.0 because 2.1.1 is compatible with 2.1.0.
gtk3 sample: remove trailing spaces
gtk3 sample: use " (double quote) for string literal
gtk3 sample: remove needless blank lines
gtk3 sample: fix indent
gtk3 sample: add missing spaces
gtk3 sample: fold long lines
gtk3 sample: use more meaningful name

w -> widget
dc (Drag Context) -> context
selectiondata -> selection_data
gtk3 sample: use "-" for word separator
gtk3 sample: use "draw" instead of "expose-event"

Because "expose-event" signal has been replaced by "draw" signal
since GTK+ 3.
gtk3 sample: use cairo_context for drawing
gtk3 sample: cleanup
Add 2.1.1 entry
gio2-gi: use '-' for word separator in path
gio2: make GI based bindings the official
gi: support freeing returned struct
gtk3: implement Gtk::SearchBar