Lua binding compatibility for lua 5.3+

[owent]

1. luaL_checkint is replaced by luaL_checkinteger and is removed from lua 5.3
2. lua_isinteger is already defined in lua 5.3+
3. else if (isprint(ch)) in upb/bindings/lua/upbc.cc is UB when ch is negative, and will crash when compiled by MSVC.