install-dependencies.sh: install add-apt-repository when it's not installed
install-dependencies.sh: install add-apt-repository when it's not installed

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1469581128-21529-2-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,cloudius-systems/seastar,syuu1228/seastar,dreamsxin/seastar,dreamsxin/seastar,dreamsxin/seastar,kjniemi/seastar,avikivity/seastar,kjniemi/seastar,scylladb/seastar,scylladb/seastar,kjniemi/seastar,syuu1228/seastar,cloudius-systems/seastar