ecl 13.5.1

The current version of Embeddable Common Lisp (ecl) in Homebrew is rather old (12.7.1, released July 2012).  Since the current version (13.5.1, released May 2013) contains many improvements and bug fixes it would be nice to have it in Homebrew by default.

I've tested the update with several large Lisp programs (ALisp, Snark theorem prover, etc.).