stdlib: require lua-stdlib 37.

Move to latest stdlib in preparation for using it's better
object system.
* rockspec.conf (dependencies): Bump stdlib to 37.
* bin/zz, bin/zmacs (require_version): Likewise.
* lib/zmacs/zlc: Rewrite option parsing around std.optparse,
instead of std.getopt.
* lib/zmacs/zlc, lib/zile/zlisp.lua: Require "io", instead of
obsolete "io_ext".
* lib/zile/lib.lua (iscallable): Now that all std.container and
std.object based tables have __call metamethods, be more careful
about detecting callable symbols.
* .travis.yml (script): Add another temporary github rockspec
while waiting for stdlib 37 to reach the LuaRocks repo.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>