ci: upgrade golang, golangci-lint, microk8s

kubectl and golangci-lint should be entirely transparent, so I won't
dwell on them too much. The big changes here are:

* Move to Go 1.13. I've been running Shipper locally in this version
already, so no issues are expected. Since Go 1.13 now validates versions, we
had to fix one issue where apimachinery had an invalid timestamp, but no
other trouble was found.

* Move to microk8s 1.15. We're confident enough that Shipper is stable
against 1.14, so let's start trying out a newer version to see how we
fare.

.travis.yml

@@ -1,18 +1,18 @@
 language: go
 go:
-- 1.12.x
+- 1.13.x
 sudo: required
 dist: xenial
 services:
 - docker
 before_install:
-- sudo snap install microk8s --classic --channel=1.14/stable
-- curl -L https://storage.googleapis.com/kubernetes-release/release/v1.15.3/bin/linux/amd64/kubectl > $(go env GOPATH)/bin/kubectl
+- sudo snap install microk8s --classic --channel=1.15/stable
+- curl -L https://storage.googleapis.com/kubernetes-release/release/v1.15.6/bin/linux/amd64/kubectl > $(go env GOPATH)/bin/kubectl
 - chmod +x $(go env GOPATH)/bin/kubectl
 - mkdir ~/.kube
 - sudo microk8s.config > ~/.kube/config
 - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh |
-  sh -s -- -b $(go env GOPATH)/bin v1.17.1
+  sh -s -- -b $(go env GOPATH)/bin v1.21.0
 - sudo microk8s.status --wait-ready
 - sudo microk8s.enable registry
 install: true

go.mod

@@ -1,6 +1,6 @@
 module github.com/bookingcom/shipper
 
-go 1.12
+go 1.13
 
 require (
 	cloud.google.com/go v0.39.0 // indirect
@@ -54,3 +54,5 @@ require (
 	k8s.io/utils v0.0.0-20190529001817-6999998975a7 // indirect
 	sigs.k8s.io/yaml v1.1.0
 )
+
+replace k8s.io/apimachinery v0.0.0-20190602113612-63a6072eb563 => k8s.io/apimachinery v0.0.0-20190602183612-63a6072eb563

go.sum

@@ -352,7 +352,6 @@ k8s.io/api v0.0.0-20190602205700-9b8cae951d65/go.mod h1:03pQc5o/sznCJkBfM5JE6PDB
 k8s.io/apiextensions-apiserver v0.0.0-20190602131520-451a9c13a3c8 h1:EqVj1O4rx5Ei3v5GZTc+koIsRr52HEbpKb3hYju+r54=
 k8s.io/apiextensions-apiserver v0.0.0-20190602131520-451a9c13a3c8/go.mod h1:vBDmZGUuVk3xyaO5Y50RRbmbzyUMs4CRtk3yhXuBteg=
 k8s.io/apimachinery v0.0.0-20190531131812-859a0ba5e71a/go.mod h1:u/2VL7tgEMV0FFTV9q0JO+7cnTsV44LP8Pmx41R4AQ4=
-k8s.io/apimachinery v0.0.0-20190602113612-63a6072eb563/go.mod h1:u/2VL7tgEMV0FFTV9q0JO+7cnTsV44LP8Pmx41R4AQ4=
 k8s.io/apimachinery v0.0.0-20190602125621-c0632ccbde11/go.mod h1:u/2VL7tgEMV0FFTV9q0JO+7cnTsV44LP8Pmx41R4AQ4=
 k8s.io/apimachinery v0.0.0-20190602183612-63a6072eb563 h1:FqK+g186du3W9peHKCV2GPqMSNEIHz1QUrfYggvaseU=
 k8s.io/apimachinery v0.0.0-20190602183612-63a6072eb563/go.mod h1:u/2VL7tgEMV0FFTV9q0JO+7cnTsV44LP8Pmx41R4AQ4=