ci/travis: fix opam install packages to locked io-page to 1.6.1

[zchee]

Update opam packages dependency to latest.
See also: https://github.com/moby/hyperkit#building

version locked io-page to 1.6.1.

[chuckyz]

@zchee I took a shot at trying to fix this yesterday but don't understand ocaml well enough to really fix the issue.

The problem is that Qcow.Make expects two objects, which you can see here:

https://github.com/moby/hyperkit/blob/master/src/lib/mirage_block_ocaml.ml#L11

[zchee]

@chuckyz Thanks trying to fix it :)
It causes is OCaml packages dependency. Docker dev changed qcow-format to qcow, that maybe include destructive change.

We have two way of fix this issue. One of use specific version of qcow-format, Two of update libhyperkit to latest(which is this projects vendor package).
However, since the second option will change the internal structure significantly, it will be necessary to update the version of docker-machine-driver-xhyve.
We need to fix travis build way and change to be able to generate prebuild binary in the current version(for next release), So I choose the former.

I will try various versions to solve OCaml's dependencies. Anyway, thanks for follow up this issue.