Check for existence of 'apps' and 'users' in the configuration file.

Fixes malomalo#4

sync-accounts

@@ -28,11 +28,16 @@ def read_permissions(url)
   end
 
   # Add keys to users and apps
-  yaml['users'].each do |user|
-    permissions['users'][user] = github_keys(user)
+  if (yaml['users'])
+    yaml['users'].each do |user|
+      permissions['users'][user] = github_keys(user)
+    end
   end
-  yaml['apps'].each do |app, users|
-    permissions['apps'][app] = users.map{|u| github_keys(u) }.flatten
+
+  if (yaml['apps'])
+    yaml['apps'].each do |app, users|
+      permissions['apps'][app] = users.map{|u| github_keys(u) }.flatten
+    end
   end
 
   permissions