Auto-DevOps.gitlab-ci.yml: Upgrade HELM_VERSION to 2.10.0

Required upgrade so that we can use "helm.sh/hook-delete-policy": before-hook-creation in charts/auto-deploy-app!7 (merged). (Introduced in Helm 2.9)

Should we decide to use Tiller TLS in Auto Devops, this will also bring in the security patch from 2.7.2.

Also brings in many fixes such as https://github.com/helm/helm/issues/1769#issuecomment-334530714

Related issue

https://gitlab.com/gitlab-org/gitlab-ce/issues/48004

Closes #80 (closed) too