Undocumented dev dependencies

[palmerev]

I tried running npm run release, but uglify-js was throwing a syntax error. I realized that I also needed babel-preset-es2015 for it to work. Then adding the preset in the release command works:
browserify lib/index.js -o dist/github-calendar.min.js -s GitHubCalendar && babel --presets es2015 dist/github-calendar.min.js | uglifyjs -c -m -o dist/github-calendar.min.js

Or, adding babel config to package.json:

"babel": {
    "presets": ["es2015"]
  }

I think it would be a good idea to have everything that the release command uses be declared in devDependencies in the package.json, something like:

"devDependencies": {
    "babel-cli": "^6.8.0",
    "babel-preset-es2015": "^6.6.0",
    "browserify": "^13.0.0",
    "uglify-js": "^2.6.2"
  }

There could also be a note in the CONTRIBUTING.md about this. 📝

[IonicaBizau]

@palmerev We should use dist-it instead. 😁 Thanks for bringing this in attention!