Helm chart release to gh-pages in Travis CI

[amimof]

- What I did

• Publish Helm chart to gh-pages branch on tagged commits
• Added additional Helm-releated tasks in Makefile

- How I did it

• Added an additional deploy task in .travis.yml that publishes to gh-pages branch on tagged commits
• Added three new Makefile targets: helm_package, helm_index and helm_lint

- How to verify it N/A

- Description for the CHANGELOG