HRR, client auth and retry are broken and will require new implementation when new stack is ready.

strace showed that the test was failing due to:

    socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP) = 5
    sendto(5, "4\320\303\231aj\5\0", 8, 0, {sa_family=AF_INET6,
        sin6_port=htons(12345), inet_pton(AF_INET6, "::1", &sin6_addr),
        sin6_flowinfo=0, sin6_scope_id=0}, 28) =
        -1 EADDRNOTAVAIL (Cannot assign requested address)

Related to https://github.com/travis-ci/travis-ci/issues/8711

Fixes #90

Add "-x sockets" option to travis script, to avoid running the socket tests known to fail in the travis environment.

add build of picotls (external lib)
build with gcc(-7) and clang(-5)