Change `abs(z) > 2` to `abs2(z) > 4` for 3x speedup

As discussed [here](https://github.com/JuliaLang/julia/pull/2974#issuecomment-17240009)
Merge pull request #2985 from karbarcca/patch-1

Change `abs(z) > 2` to `abs2(z) > 4` for 3x speedup