golang: v↑ go1.13 (1.13.9 -> 1.13.15)

Going Go1.13.9 -> Go1.13.15 brings in many compiler and runtime fixes,
including security fixes to package net/http:

https://golang.org/doc/devel/release.html#go1.13

Switch helloweb from go1.12 -> go1.13 to test.

Tested on helloworld SR.

component/golang/buildout.cfg

@@ -70,8 +70,8 @@ environment-extra =
 
 [golang1.13]
 <= golang-common
-url = https://dl.google.com/go/go1.13.9.src.tar.gz
-md5sum = 4ad8b04f962be93a32f3021e6f35b3b9
+url = https://golang.org/dl/go1.13.15.src.tar.gz
+md5sum = 4f4af14d88352a62761a9dcedf863ac0
 
 # go1.13 needs go1.4 to bootstrap
 environment-extra =

component/helloweb/buildout.cfg

@@ -25,7 +25,7 @@ parts =
 install =
     lab.nexedi.com/nexedi/helloweb/go/...
 
-golang  = ${golang1.12:location}
+golang  = ${golang1.13:location}
 
 # -*- go -*-
 [helloweb-go]