use Python 2.6 style format for #5

Author: defrex

encrypted_fields/fields.py

@@ -188,7 +188,7 @@ def get_db_prep_value(self, value, connection, prepared=False):
                     and len(value) > self.max_length
                 ):
                     raise ValueError(
-                        'Field {} max_length={} encrypted_len={}'.format(
+                        'Field {0} max_length={1} encrypted_len={2}'.format(
                             self.name,
                             self.max_length,
                             len(value),