Update/fix aptly debian repository GPG key

Change-Id: I59b38dd22c06d6872852697d718d925bd16fa28c
See: https://www.aptly.info/download/
diff --git a/linux/system/repo/aptly.yml b/linux/system/repo/aptly.yml
index 330ba03..87d816d 100644
--- a/linux/system/repo/aptly.yml
+++ b/linux/system/repo/aptly.yml
@@ -5,5 +5,5 @@
         aptly:
           source: "deb http://repo.aptly.info/ squeeze main"
           architectures: amd64
-          key_id: 9E3E53F19C7DE460
-          key_server: keys.gnupg.net
+          key_id: ED75B5A4483DA07C
+          key_server: pool.sks-keyservers.net