test: bump --stack-size to stop child fails on ARM

On ARM, we get a "Maximum call stack size exceeded" when using
require() in the child process, bump it up a bit to avoid the
failures so we can test what we are actually after

Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/iojs/io.js/pull/14