Upgrade certificates dependency.

go.mod

@@ -5,10 +5,9 @@ go 1.13
 require (
 	github.com/go-logr/logr v0.3.0
 	github.com/jetstack/cert-manager v1.3.1
-	github.com/mattn/go-colorable v0.1.8 // indirect
 	github.com/onsi/ginkgo v1.14.1
 	github.com/onsi/gomega v1.10.2
-	github.com/smallstep/certificates v0.15.15
+	github.com/smallstep/certificates v0.17.1
 	k8s.io/api v0.20.2
 	k8s.io/apiextensions-apiserver v0.20.2 // indirect
 	k8s.io/apimachinery v0.20.2