package.json: Add snap build for Linux.

[flexiondotorg]

Hey @andrepolischuk I work on the Snapcraft team and would love to help get Keep published in the Snap Store, which is the app store for Linux :smiley: This pull request adds a snap to the Linux builds and bumps the electron-builder version which provides improved snap support.

Once published, you'll have direct control of the publishing life cycle, access to metrics for weekly active devices and users will get automatic updates. I see you're using Travis, here are the docs for publishing in the Snap Store.

• https://docs.snapcraft.io/releasing-to-the-snap-store/6848
• https://docs.travis-ci.com/user/deployment/snaps/

I'd love to feature Keep as an editors pick in the store and put together a social campaign to promote it. Is there any other help you need to get Keep published in the snap store?