usb-native v6.0.0

node-usb-native

1. Update electron version in index.js
2. Update electron version in ./scripts/rebuild-serialport.js node-gyp rebuild --target=[new-electron-version] --arch=x64 --dist-url=https://atom.io/download/electron
3. Run Travis CI job to compile cross-platform libraries for the electron version
4. Get new native binaries from https://github.com/VSChina/serialport.node/releases.
5. Place the new libraries under /native folder and remove the useless libraries to reduce package size.
6. Update package version number and run npm publish ro publish the new version.