Enable automatic snap build and release

[fcole90]

Now the snap can be built and released by CI like the gem

[fcole90]

In order for the deploy step to work, you need to save your Snap Store token in a Travis environment variable named SNAP_TOKEN containing your token.

You can get your token with

snapcraft export-login --snaps travis --channels stable -

And then all should work fine and keep travis updated on the store

[fcole90]

Can someone merge this, given that it's now approved?