Don't enable pylab mode, when matplotlib is not importable

On a headless server, matplotlib is a unnecessary dependency and the ipython
console will crash badly, when tring to enable %pylab there.
In that case, just don't enable %pylab and ask the user to install matplotlib.

Original bug report: https://bugzilla.redhat.com/show_bug.cgi?id=872176
Merge pull request #2681 from tomspur/remove_matplotlib_dep

Don't enable pylab mode, when matplotlib is not importable