Fix electron-rebuild for Electron 3
Fix electron-rebuild for Electron 3

This tiny change fixes
```
\node_modules\opencv4nodejs\cc\opencv4nodejs.cc(63): error C2653: 'Core': is not a class or namespace name
\node_modules\opencv4nodejs\cc\opencv4nodejs.cc(63): error C3861: 'Init': identifier not found 
```
when running `electron-rebuild` with `electron@3.0.0-beta.2`.
justadudewhohacks/opencv4nodejs,justadudewhohacks/opencv4nodejs,justadudewhohacks/opencv4nodejs,justadudewhohacks/opencv4nodejs,justadudewhohacks/opencv4nodejs,justadudewhohacks/opencv4nodejs