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

io-page 2.0.0 (which required by mirage-block-unix) is no compatible
to qcow-format 0.5.0. io-page package to version locked to 1.6.1.

Signed-off-by: Koichi Shiraishi <zchee.io@gmail.com>

Author: zchee

.travis.yml

@@ -1,5 +1,3 @@
-sudo: false
-
 os: osx
 osx_image: xcode8
 
@@ -14,14 +12,16 @@ env:
     - HOMEBREW_NO_AUTO_UPDATE=1
     - MAKEFLAGS="-j$(sysctl -n hw.ncpu)"
     - RELEASE_GO_VERSION=''
+    - OPAMYES=1
+    - V=1
 
 before_install:
   - uname -a
   - env | sort
   # for ocaml-qcow
   - brew install https://github.com/Homebrew/homebrew-core/raw/master/Formula/opam.rb
   - brew install https://github.com/Homebrew/homebrew-core/raw/master/Formula/libev.rb
-  - opam --version && opam init --yes && opam install --yes uri qcow-format ocamlfind conf-libev
+  - opam --version && opam init && opam install uri qcow-format io-page.1.6.1 ocamlfind conf-libev
   - eval `opam config env`
 
 install: