build: We can now use Go 1.13

go.mod

@@ -52,7 +52,7 @@ require (
 	gopkg.in/ldap.v2 v2.5.1
 )
 
-go 1.12
+go 1.13
 
 // https://github.com/spaolacci/murmur3/pull/30
 replace github.com/spaolacci/murmur3 v1.1.0 => github.com/calmh/murmur3 v1.1.1-0.20200226160057-74e9af8f47ac