Add support for Lua 5.4

[alerque]

In my experience (as a dependency of yaml) this works fine under Lua 5.4, but to get it one has to install via a side channel of some kind. Direct installation via luarocks install lub is blocked because of this version restriction:

lub/lub-1.1.0-2.rockspec

Line 26 in 4b5ee46

"lua >= 5.1, < 5.4",

I think this top end restriction should just be dropped and a new rockspec released and published on LuaRocks.org so that existing users can go on using it as usual without kludges.