Implement protocol version so that we can have mixed cluster during u…

…pgrade (#55)

Signed-off-by: Jan Kryl <jan.kryl@cloudbyte.com>

.travis.yml

@@ -10,13 +10,18 @@ env:
     - ZFS_BUILD_TAGS=0
     - ZFS_BUILD_TAGS=1
 before_install:
-    # sudo apt-get -qq update
+    - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+    - sudo apt-get update -qq
+    - sudo apt-get install --yes -qq gcc-6 g++-6
     - sudo apt-get install --yes -qq build-essential autoconf libtool gawk alien fakeroot linux-headers-$(uname -r) libaio-dev
     - sudo apt-get install --yes -qq zlib1g-dev uuid-dev libattr1-dev libblkid-dev libselinux-dev libudev-dev libssl-dev
     - sudo apt-get install --yes -qq lcov libjemalloc-dev
     # packages for tests
     - sudo apt-get install --yes -qq parted lsscsi ksh attr acl nfs-kernel-server fio
     - sudo apt-get install --yes -qq libgtest-dev cmake
+    # use gcc-6 by default
+    - sudo unlink /usr/bin/gcc && sudo ln -s /usr/bin/gcc-6 /usr/bin/gcc
+    - sudo unlink /usr/bin/g++ && sudo ln -s /usr/bin/g++-6 /usr/bin/g++
 install:
     - pushd .
     - cd /usr/src/gtest