don't allow unicode errors in refill_readline_hist

If a recent session had unicode input, and the current
session has a different encoding, refill_readline_hist could raise a UnicodeEncodeError exception, preventing 
IPython from starting.  'replace' is now used when encoding the history, so this error is no longer
possible.

IPython/core/interactiveshell.py

@@ -1596,7 +1596,7 b' class InteractiveShell(Configurable, Magic):'
                                                         include_latest=True):
             if cell.strip(): # Ignore blank lines
                 for line in cell.splitlines():
-                    self.readline.add_history(line.encode(stdin_encoding))
+                    self.readline.add_history(line.encode(stdin_encoding, 'replace'))
 
     def set_next_input(self, s):
         """ Sets the 'default' input string for the next command line.