Add x86 binaries

We [need to](https://github.com/ReactTraining/react-stdio/blob/5fe65b6ef8eaffc40eec1034f07ecd743e25a43c/scripts/build.sh#L7) setup our CI environment to properly be able to build x86 binaries. [This comment](https://github.com/zeit/pkg/issues/310#issuecomment-362942350) should help.