Bump to go 1.13

.travis.yml

@@ -3,7 +3,7 @@ dist: xenial
 language: go
 
 go:
-- "1.12.x"
+- "1.13.x"
 go_import_path: github.com/coreos/kube-prometheus
 
 cache:

go.mod

 module github.com/coreos/kube-prometheus
 
-go 1.12
+go 1.13
 
 require (
 	github.com/Jeffail/gabs v1.2.0