fix IPython deprecation warning in 1.0+

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.