fix bugs and 2.6 compatibility
diff --git a/wally/pretty_yaml.py b/wally/pretty_yaml.py
index 4fc4a49..f078ff2 100644
--- a/wally/pretty_yaml.py
+++ b/wally/pretty_yaml.py
@@ -41,9 +41,9 @@
     if isinstance(data, (list, tuple)):
         if all(map(is_simple, data)):
             if all_nums(data):
-                one_line = "[{}]".format(", ".join(map(dumps_simple, data)))
+                one_line = "[{0}]".format(", ".join(map(dumps_simple, data)))
             else:
-                one_line = "[{}]".format(",".join(map(dumps_simple, data)))
+                one_line = "[{0}]".format(",".join(map(dumps_simple, data)))
         else:
             one_line = None