Changed promu and release building to Go 1.13.1 as well. (#1622)

Updated docs.

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

.circleci/config.yml

@@ -80,7 +80,7 @@ jobs:
   publish_release:
     docker:
       # Available from https://hub.docker.com/r/circleci/golang/
-      - image: circleci/golang:1.12.5
+      - image: circleci/golang:1.13.1
     working_directory: /go/src/github.com/thanos-io/thanos
     environment:
       GOBIN: "/go/bin"

.promu.yml

@@ -1,5 +1,5 @@
 go:
-    version: 1.12.5
+    version: 1.13.1
 repository:
     path: github.com/thanos-io/thanos
 build:

docs/contributing/how-to-change-go-version.md

@@ -12,5 +12,7 @@ To update Thanos build system to newer Golang:
 
 1. Edit [.promu.yaml](/.promu.yml) and edit `go: version: <go version>` in YAML to desired version. This will ensure that all artifacts are 
   built with desired Golang version. How to verify? Download tarball, unpack and invoke `thanos --version`
-1. Edit [.circleci/config.yaml](/.circleci/config.yml) and edit ` - image: circleci/golang:<go version>` to desired 
-  Golang version. This will ensure that all docker images and go tests are using desired Golang version. How to verify? Invoke `docker pull improbable/thanos:<version> --version`
+1. Edit [.circleci/config.yaml](/.circleci/config.yml) and update ` - image: circleci/golang:<go version>` to desired 
+  Golang version. This will ensure that all docker images and go tests are using desired Golang version. How to verify? Invoke `docker pull quay.io/thanos/thanos:<version> --version`
+1. Edit [.Dockerfile.thanos-ci](/Dockerfile.thanos-ci) and update Go version. Run `make docker-ci DOCKER_CI_TAG=<new tag>`. Update [.circleci/config.yaml](/.circleci/config.yml) thanos-ci image to `<new tag>`.
+