[opam-devel] travis opam now tests 4.02.0 as well

[Anil Madhavapeddy]

I have refreshed my OCaml 4.02.0 PPA to the latest snapshot as of today, but we can't use it yet since there's no version of OPAM that easily compiles with it.  OPAM 1.1 has the binary blob problem, and OPAM 1.2dev is currently broken (no base-* installed).

For now, we just `opam switch 4.02.0dev+trunk` which works fine, and 4.02 is listed as an "allowed failure" so it doesn't hold up the other merges.

See https://travis-ci.org/ocaml/opam-repository/builds/24352557 for an example build using ppx_protobuf.

-anil