Fixed mpl 1.3.1 issue with derived_phenomena example

```cont = iris.quickplot.contourf(cube)``` returns a tuple ```(<matplotlib.colorbar.Colorbar instance at 0x10fd15a28>, <matplotlib.axes.AxesSubplot object at 0x10fcfcc10>)``` under mpl 1.2.1, but just the colorbar object ```<matplotlib.colorbar.Colorbar instance at 0x1124819e0>``` under mpl 1.3.1. This workaround in the derived_phonomena.py example ensures the code works with both versions. Note there may be a image change when we bump to mpl 1.3.1, but that can be handled separately.