Raise default glusterfs version to 3.8

Change-Id: Ibafd332e616a1f3efacfccf67aaac8dd16ca8037
diff --git a/aptly/server/mirror/ubuntu/trusty/glusterfs.yml b/aptly/server/mirror/ubuntu/trusty/glusterfs.yml
index b83c49f..4d27bf6 100644
--- a/aptly/server/mirror/ubuntu/trusty/glusterfs.yml
+++ b/aptly/server/mirror/ubuntu/trusty/glusterfs.yml
@@ -1,9 +1,11 @@
 parameters:
+  _param:
+    glusterfs_version: 3.8
   aptly:
     server:
       mirror:
         glusterfs_upstream_trusty:
-          source: http://ppa.launchpad.net/gluster/glusterfs-3.7/ubuntu
+          source: http://ppa.launchpad.net/gluster/glusterfs-${_param:glusterfs_version}/ubuntu
           distribution: trusty
           components: main
           architectures: amd64