Merge pull request #6 from phi-gamma/master

2 one-line fixes to modutils.lua
Fixing package loading for LuaTeX > 0.74

Starting from LuaTeX 0.74, Lua 5.2 is used, and packages.loaders
is renamed into packages.searchers, which makes luatexbase and
thus almost all LuaLaTeX packages broken.

This commit fixes it, keeping backward compatibility.
Fixing bug by making optional argument optional
update mcb.lua for 5.2