maint: preliminary Lua 5.3.0 compatibility.

* slingshot: Sync with upstream for Lua 5.3 compatibility.
* configure.ac (AX_PROG_LUA): Accept Lua 5.3 interpreters.
* ext/yaml/lua52compat.h: Remove.
* ext/yaml/lyaml.h (lua_objlen, lua_strlen, luaL_openlib)
(luaL_register): Lua 5.2 and 5.3 compatibility definitions.
* .travis.yml: Regenerate.

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