[TravisCI] install aspcud for opam

to avoid protential Stack overflow during `opam update`

.travis.yml

@@ -42,6 +42,7 @@ install_linux: &install_linux
       ocaml
       ocaml-native-compilers
       ocaml-findlib
+      aspcud
       libpcre3-dev
       zlib1g-dev
       libgtk2.0-dev