Publish a snap package

[j0npulsifer]

Related to #7

Ref https://snapcraft.io/first-snap/golang

We should publish this as a snap such that we can snap install ejson2env. Travis can probably do the thing for us.

Here's the snapcraft.yaml that I used to build this in a pinch:

name: ejson2env
version: '1.0'
summary: ejson2env is based on the ejson library and extends the ejson file format.
description: |
  ejson2env is a tool to simplify storing secrets that should be accessible in the shell environment in your git repo.
confinement: devmode
base: core18
parts:
  ejson2env:
    plugin: go
    go-importpath: github.com/Shopify/ejson2env
    source: https://github.com/Shopify/ejson2env
    source-type: git
apps:
  ejson2env:
    command: bin/ejson2env

[thepwagner]

This is stale, we are not going to do it.