Fall back to 1 when revising a version.

In my case I've added Versioning at a later point and old document had no version field
as this field would be only populated on creating the document

lib/mongoid/versioning.rb

@@ -28,7 +28,7 @@ def revise
       if last_version
         self.versions << last_version.clone
         self.versions.shift if self.class.version_max.present? && self.versions.length > self.class.version_max
-        self.version = version + 1
+        self.version = (version || 1 ) + 1
         @modifications["versions"] = [ nil, @attributes["versions"] ] if @modifications
       end
     end