update travis scripts

Don’t support ocaml < 4.01.0. Use new opam binaries now available for
Ubuntu.
Introduced module Result_stream

This module is meant to use Result (or Or_error) stream more easily:
functions passed to [map] or [fold] do not need to do the pattern
matching on the [Result] type. Moreover, [fold] functions stop their
computation as soon as they encounter an error, contrary to
[Stream.fold].
Result_stream to Stream.Result

- moved the contents of Result_stream to Stream.Result
- added Stream.Or_error
- renamed {map, fold} functions to {map', fold'} and vice-versa