bump go version in go.mod (#6194)

Signed-off-by: Tariq Ibrahim <tariq181290@gmail.com>

go.mod

@@ -1,8 +1,6 @@
 module github.com/prometheus/prometheus
 
-go 1.12
-
-replace k8s.io/klog => github.com/simonpasquier/klog-gokit v0.1.0
+go 1.13
 
 require (
 	cloud.google.com/go v0.44.1 // indirect
@@ -83,4 +81,7 @@ require (
 	k8s.io/utils v0.0.0-20190809000727-6c36bc71fc4a // indirect
 )
 
-replace github.com/golang/lint => golang.org/x/lint v0.0.0-20190409202823-959b441ac422
+replace (
+	github.com/golang/lint => golang.org/x/lint v0.0.0-20190409202823-959b441ac422
+	k8s.io/klog => github.com/simonpasquier/klog-gokit v0.1.0
+)