override vformat instead of _vformat

in our eval formatters

avoids being caught out by changes in private API

this needs to be backported in order to support Python 3.4.4

IPython/utils/text.py

@@ -540,9 +540,7 b' class FullEvalFormatter(Formatter):'
     """
     # copied from Formatter._vformat with minor changes to allow eval
     # and replace the format_spec code with slicing
-    def _vformat(self, format_string, args, kwargs, used_args, recursion_depth):
-        if recursion_depth < 0:
-            raise ValueError('Max string recursion exceeded')
+    def vformat(self, format_string, args, kwargs):
         result = []
         for literal_text, field_name, format_spec, conversion in \
                 self.parse(format_string):