Fix GTK Version in .travis.yml globally
Fix GTK Version in .travis.yml globally

insanebits/gtk,LordSentox/gtk,Susurrus/gtk,drewet/gtk,rust-gnome/gtk,gkoz/gtk-rs-pango,EPashkin/rust-gnome-glib,gkoz/gtk-rs-glib,rtsuk/cairo,Susurrus/gtk,gkoz/gtk-rs-gtk,daa84/gtk,EPashkin/rust-gnome-gtk,Susurrus/glib,gkoz/gtk-rs-cairo,gkoz/gtk-rs-gtk,rust-gnome/gtk,gkoz/rust-gnome-cairo,mitaa/gtk,gkoz/rust-gnome-gtk,rust-gnome/glib,daa84/gtk,nicokoch/glib,gkoz/rust-gnome-glib,LordSentox/gtk,insanebits/gtk,rust-gnome/cairo,EPashkin/rust-gnome-gtk,drewet/gtk,mitaa/gtk,gkoz/rust-gnome-gtk