release-tools: update

Commit summary:
82d108a switch to Go 1.15
843bddc Add steps on promoting release images

release-tools/SIDECAR_RELEASE_PROCESS.md

@@ -86,6 +86,12 @@ naming convention `<hostpath-deployment-version>-on-<kubernetes-version>`.
    [external-provisioner example](https://github.com/kubernetes-csi/external-provisioner/releases/new)
 1. If release was a new major/minor version, create a new `release-<minor>`
    branch at that commit.
+1. Check [image build status](https://k8s-testgrid.appspot.com/sig-storage-image-build).
+1. Promote images from k8s-staging-sig-storage to k8s.gcr.io/sig-storage. From
+   the [k8s image
+   repo](https://github.com/kubernetes/k8s.io/tree/master/k8s.gcr.io/images/k8s-staging-sig-storage),
+   run `./generate.sh > images.yaml`, and send a PR with the updated images.
+   Once merged, the image promoter will copy the images from staging to prod.
 1. Update [kubernetes-csi/docs](https://github.com/kubernetes-csi/docs) sidecar
    and feature pages with the new released version.
 1. After all the sidecars have been released, update

release-tools/travis.yml

@@ -6,7 +6,7 @@ git:
   depth: false
 matrix:
   include:
-  - go: 1.13.3
+  - go: 1.15
 before_script:
 - mkdir -p bin
 - wget https://github.com/golang/dep/releases/download/v0.5.1/dep-linux-amd64 -O bin/dep