fix upgrading from old plugin config files

src/plugins/plugin.py

@@ -264,9 +264,10 @@ class GajimPluginConfig():
                         '%s and creating a new one' % (self.plugin.short_name,
                         self.FILE_PATH + '.bak'))
                     if os.path.exists(self.FILE_PATH + '.bak'):
-                        os.rename(self.FILE_PATH, self.FILE_PATH + '.bak')
-                        self.data = {}
-                        self.save()
+                        os.remove(self.FILE_PATH + '.bak')
+                    os.rename(self.FILE_PATH, self.FILE_PATH + '.bak')
+                    self.data = {}
+                    self.save()
         else:
             self.data = {}
             self.save()