form key ordering: detect django version

- Django 1.6 and before use SortedDict instead of OrderedDict
  It detect version and return proper dict for key ordering.
- field deletion is done by reorder funcion.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

symposion/forms.py

@@ -1,6 +1,7 @@
 from collections import OrderedDict
 
 from django import forms
+from django import VERSION as django_VERSION
 
 import account.forms
 
@@ -13,7 +14,6 @@ class SignupForm(account.forms.SignupForm):
 
     def __init__(self, *args, **kwargs):
         super(SignupForm, self).__init__(*args, **kwargs)
-        del self.fields["username"]
         key_order = [
             "email",
             "email_confirm",
@@ -46,4 +46,10 @@ def reorder_fields(fields, order):
         if key not in order:
             del fields[key]
 
-    return OrderedDict(sorted(fields.items(), key=lambda k: order.index(k[0])))
+    if django_VERSION < (1, 7, 0):
+        # fields is SortedDict
+        fields.keyOrder.sort(key=lambda k: order.index(k[0]))
+        return fields
+    else:
+        # fields is OrderedDict
+        return OrderedDict(sorted(fields.items(), key=lambda k: order.index(k[0])))