Build: upgrade Go to 1.13.1

.circleci/config.yml

@@ -62,7 +62,7 @@ jobs:
     machine:
       image: ubuntu-1604:201903-01
     environment:
-      GO_VERSION: 1.12.5
+      GO_VERSION: 1.13.1
       # We don't need a GOPATH but CircleCI defines it, so we override it
       GOPATH: /home/circleci/go
       GO111MODULE: 'on'

docs/contributing/building.md

@@ -1,6 +1,6 @@
 # Building
 
-You'll need a working `go` environment version >= 1.11 (official releases are built against `1.12`).
+You'll need a working `go` environment version >= 1.11 (official releases are built against `1.13`).
 It's also expected that you have a Docker daemon for building images.
 
 Clone the repository. The project uses [Go Modules](https://github.com/golang/go/wiki/Modules),

go.mod

@@ -1,6 +1,6 @@
 module github.com/fluxcd/flux
 
-go 1.12
+go 1.13
 
 require (
 	github.com/Jeffail/gabs v1.4.0

snap/snapcraft.yaml

@@ -46,7 +46,7 @@ parts:
       - gcc
       - git
     build-snaps:
-      - go/1.12/stable
+      - go/1.13/stable
     stage:
       - -bin/fluxd
       - -bin/helm-operator