Add PHP 5.6 Formula

I'm not sure if you take beta builds or not, but I wanted to try out PHP 5.6 and variadics, and figured that this would be added eventually anyways so might as well get it started.

One thing to note though, I can only get this to build when using `--HEAD`, and even then only after force linking `bison`. I'm not sure how to specify the bison version to use while compiling within the formula.