Alternative solution for julian (ch06-03)

I found the current approach to `julian/5` to be a bit difficult to explain and understand.

This feels much more natural to me (but, admittedly, I'm only a year into playing with Erlang, so take that with a grain of salt).

It reflects the style advocated by @gar1t in his blog post [Solving Embarrassingly Obvious Problems In Erlang](http://www.gar1t.com/blog/2012/06/10/solving-embarrassingly-obvious-problems-in-erlang/), which doesn't fit all that well with other examples given.

**Note**: there's also a bug fix buried in here: the last `integer` in the argument list in the spec for `julian/5` should be `integer()` for dialyzer to work properly.