fix initial sys.path

recent change caused initial sys.path to lose '' as the first element in the notebook.

closes  #4317

IPython/core/application.py

@@ -137,7 +137,7 b' class BaseIPythonApplication(Application):'
     )
     def _ipython_dir_default(self):
         d = get_ipython_dir()
-        self._ipython_dir_changed('ipython_dir', '', d)
+        self._ipython_dir_changed('ipython_dir', d, d)
         return d
     
     _in_init_profile_dir = False