Prevent logger from logging twice in IPython notebooks

The astropy logger will log twice in IPython notebooks due to the fact that the logger is being propagated to the IPython logger. This results in things like this:

http://nbviewer.ipython.org/gist/jzuhone/4d4d315dceecf658eb57

where the IPython logger is highlighted in pink/red/something like that.

The small change below resolves this behavior, which results in this:

http://nbviewer.ipython.org/gist/jzuhone/8d387639a530418594df

which is behavior consistent with the console.