Publish to the snap store from Travis

.travis.yml

sudo: required
dist: trusty
env:
    global:
        secure: <redacted>
services:
    - docker
language: python
python:
    - '3.5'
install:
    - sudo add-apt-repository ppa:facundo/salty -y
    - sudo apt-get update -q
    - sudo apt-get install libsodium13 -y
    - pip3 install snapstore-publish
script:
    - docker run -v $(pwd):/snapcraft didrocks/snapcraft sh -c 'cd /snapcraft; snapcraft'
cache:
    - "$HOME/.snapstore-publish-macaroon.enc"
after_success:
    - "snapstore-publish --name <your-snap> --series 16 --channel beta *.snap"