Update Helm to 2.15.2 for the 'gitlab-charts-build-base' image

Signed-off-by: Rémy Coutable <remy@rymai.me>

Dockerfile.gitlab-charts-build-base

 FROM ruby:2.4.5-alpine
 MAINTAINER GitLab Distribution Team
 
-ENV HELM_VERSION=2.14.3
+ENV HELM_VERSION=2.15.2
 ENV HELM_URL=https://kubernetes-helm.storage.googleapis.com/helm-v${HELM_VERSION}-linux-amd64.tar.gz
 
 # kubectl (possibly in gcloud?)