Security update for libyaml

libyaml updated to upstream 0.1.6 (hg bce8b60f0b9a) in order to
fix heap overflow in yaml_parser_scan_uri_escapes.

This patch also revert our hack with extra identation of
flow-style sequences and maps at root level:

    setmetatable({10, 15, 20}, { __serialize="seq"})
    now encoded as
    "--- [10, 15, 20]\n...\n"
    instead of
    "---\n[10, 15, 20]\n...\n".

This trick is no more needed because REPL never uses flow mode
at root level.