Support snap packaging

[3v1n0]

It would be nice if you could support http://snapcraft.io/ and autouploads to the store.

Since this is based on electron-builder, it's quite easy to support it.
See https://github.com/electron-userland/electron-builder/blob/master/docs/configuration/snap.md

[z-------------]

Snap packaging is something that I plan to support at some point, but there is an issue building snaps on Travis's Trusty images (cf. #27) that I haven't yet gotten around to dealing with.

[holazt]

@z------------- Now Travis-ci support Ubuntu 16.04 images.

[3v1n0]

Also without that it was quite easy to use a docker image (snapcraft image) from an ubuntu 14.04 ubuntu image, so it wasn't really a blocker.