Update go.mod to "1.13"

This is part of a consistency exercise for #33 - the go mod
version now matches the Go version in the Dockerfile.

This was queried with @bmcstdio, but we got no response on his
PR.

#24 (comment)

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Author: alexellis

template/golang-http-armhf/go.mod

@@ -1,6 +1,6 @@
 module handler
 
-go 1.12
+go 1.13
 
 replace handler/function => ./function
 

template/golang-http/go.mod

@@ -1,6 +1,6 @@
 module handler
 
-go 1.12
+go 1.13
 
 replace handler/function => ./function
 

template/golang-middleware-armhf/go.mod

@@ -1,5 +1,5 @@
 module handler
 
-go 1.12
+go 1.13
 
 replace handler/function => ./function

template/golang-middleware/go.mod

@@ -1,5 +1,5 @@
 module handler
 
-go 1.12
+go 1.13
 
 replace handler/function => ./function