ltj-base.lua: use precompiled cache (.luc, .lub) if possible, as luaotfload

ltj-base.lua: use precompiled cache (.luc, .lub) if possible, as luaotfload

I tested with the manual of LuaTeX-ja:

                   .luc     .lua
(Japanese manual) 10.446s  10.624s
(English manual)   8.565s   8.723s

Used LuaTeX:
  > This is LuaTeX, Version beta-0.76.0-2013061708  (TeX Live 2013) (rev 4627)