MAJOR: go-version: update Go min. version (#43)

amelhusic · web-flow · commit faede19a4e46 · 2020-04-28T11:20:03.000+02:00
with 6a6fdaa commit Milliseconds() is used. Since it is a Go 1.13 feature, Go min. version is updated from 1.12 to 1.13 in go.mod file
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/haproxytech/haproxy-consul-connect
 
-go 1.12
+go 1.13
 
 require (
 	github.com/asaskevich/govalidator v0.0.0-20190424111038-f61b66f89f4a // indirect
