Make ob-sml work with the package manager in E24

Wanted to use the E24 package for ob-sml.

So:

* Added a package standard version header and file-end. 
* Set required sml-mode to 6.0, it is the oldest version available on ELPA.
* Tested installation of it locally on E24. 

It install just fine.

Something isn't working when I evaluate, but I'll look at that later.