var Nightmare = require('nightmare');
var nightmare = Nightmare({ show: true , webPreferences:{ partition: 'nopersist' }});
Erectronのドキュメントより
https://github.com/electron/electron/blob/master/docs/api/browser-window.md#new-browserwindowoptions
↓のoption名 'web-preferences'は古いものなようでハマった・・・
http://stackoverflow.com/questions/33944837/clear-cache-in-nightmare-js-electron