Force use of gpg2.

Change-Id: I393dd410f5d46f0918c9a379d28171e91952627e
diff --git a/mendel-builder/aptly.conf b/mendel-builder/aptly.conf
index 15de27a..aa60d3d 100644
--- a/mendel-builder/aptly.conf
+++ b/mendel-builder/aptly.conf
@@ -1,5 +1,6 @@
 {
   "rootDir": "/var/lib/aptly",
+  "gpgProvider": "gpg2",
   "FileSystemPublishEndpoints": {
     "unstable": {
       "rootDir": "/var/lib/aptly/publishes/unstable",