fix rationalize (#5935)

Potential fix for #5935. Travis failure likely to be #8200.

cc @rickhg12hs @StefanKarpinski 
Ok. I thought that was covered by [other](https://github.com/JuliaLang/julia/blob/master/base/int.jl#L589) [comments](https://github.com/JuliaLang/julia/blob/master/base/int.jl#L600), but i see what you mean now.