fix IPython deprecation warning in 1.0+

Hi Andreas and Aaron! hope this drive-by patch finds you in good health and in
good spirits.


Prior to this commit, users of IPython 1.0 and higher would be greeted with a
deprecation warning like this:

    /home/pi/code/ipython/IPython/frontend.py:30: UserWarning: The
   top-level `frontend` package has been deprecated. All its
   subpackages have been moved to the top `IPython` level.

This commit addresses that for newer version of IPython, preserving the old
behavior if importing from the new location fails.