opam, travis: remove cfstream dependency

Should have been done with ee50b2ff2998fe4c78ebd3316993fe91bec76d8c.
opam: require core >= 111.17.00

Should have been done with aa7027444d7d9c10635c4c70b6b1b0646a65165c.
OMakeroot: lwt & async lib install now optional

Although we had compilation of future.lwt and future.async optional,
the install target assumed they existed. It would then fail if the
corresponding files didn’t exist. Now using ocamlfind’s -optional
argument to handle correctly.