Support sprockets up to 3.0
Merge pull request #8 from mirego/update/support-sprockets-before-3.0

Support sprockets up to 3.0
Remove Heroku “labs” feature in README
Merge pull request #9 from mirego/feature/remove-heroku-labs-note-in-readme

Remove Heroku “labs” feature in README
Update dependencies and add Rubocop and Phare