Added lua heredoc support

With the rise of lua as an embedded scripting language, i.e. in Redis, I found myself writing small lua scripts in my Ruby applications.

This little update gives those heredocs a little make over.

```ruby
<<-LUA
  local retval,j
  retval={}
  for i=1,#KEYS do
    table.insert(retval,redis.call('exists',KEYS[i]))
  end
  return retval
LUA
```

Cheers,

Lukas