Use stdio instead of customFds

This fixes the noisy deprecation warnings when node-gyp is used with >= node v0.11.14. Apparently the `stdio` option has been around and `customFds` has been deprecated since node v0.5.11 (https://github.com/joyent/node/commit/245469587c7a6326d1b55bdf6c6d6650d72bfa22).

 - refs joyent/node#8492

Would love to see this land since for node-pre-gyp I test expected stdout and stderr and therefore this deprecation warning breaks my tests against node v0.11.14: https://travis-ci.org/mapbox/node-pre-gyp/builds/40707288