Fix compilation of the ssl backend

[samoht]

Also complete the matrix in the travis script so that future regressions will be caught faster.

[rgrinberg]

Is this a complete matrix though? We should also test the dummy modules i.e. whenever ssl or async_ssl are absent but conduit-lwt-unix or conduit-async are built.

[samoht]

yes, it's what DEPOPTS is supposed to do: first build without extra packages, then install the depopts. I'll double check that's it's indeed what happens.

[samoht]

So it's compiling both with and without the all the depopts. So I should probably try to alternate between tls/ssl/both. Also it seems that the tests are not run (very simple tests atm, just trying to link stuff together) so I am trying to fix that.

[samoht]

Tests are not running because we need a new release of ocaml-ci-scripts, which is now in progress: ocaml/ocaml-ci-scripts#179

[samoht]

So it appears that fixing the CI is much more involved that I was planning. Given that the current released version of conduit is broken, I'll focus now on making a new release - and I (or someone else) could fix the CI later.