[TravisCI] install libgtk2.0-dev for building neko

.travis.yml

@@ -36,6 +36,7 @@ install_linux: &install_linux
       camlp4
       libpcre3-dev
       zlib1g-dev
+      libgtk2.0-dev
       awscli
   - wget https://raw.github.com/ocaml/opam/master/shell/opam_installer.sh -O - | sh -s /usr/local/bin system
   - export OPAMYES=1