Extend Travis CI to cover Clang 5.0 builds.
Extend Travis CI to cover Clang 5.0 builds.

Signed-off-by: Vladimir Krivopalov <8393b6f566d8ebdefd4d2517fe16f44423413d9c@gmail.com>
Message-Id: <20171125034448.1850-1-8393b6f566d8ebdefd4d2517fe16f44423413d9c@gmail.com>

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