doc: expliclty 'set +ex' to avoid travis leaking secure variables
doc: expliclty 'set +ex' to avoid travis leaking secure variables

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

mirage/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,samoht/ocaml-vchan,mirage/ocaml-vchan,samoht/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan