Also build packages using certain ocamlbuild plugins on CI.

These haven't been broken in the past, and let's keep it this way.

Author: whitequark

.travis.yml

@@ -25,3 +25,5 @@ script:
   - opam install -v ocamlbuild
   - opam install -v mtime # this tests topkg, with stub libraries
   - opam install -v inotify # this tests oasis, with stub libraries
+  - opam install -v cpuid # this tests the ocb-stubblr plugin
+  - opam install -v ppx_deriving # this tests the cppo plugin