Iris v1.6 whatsnew.
Merge pull request #917 from bjlittle/whats_new_1p6

Iris release v1.6 whatsnew.
Fixed issue with derived_phenomena example expecting colorbar object to
be a tuple.
Merge pull request #952 from esc24/derived_phenom_example_mpl131_support

Fixed mpl 1.3.1 issue with derived_phenomena example
Test images updated to reflect changes in MPL v1.3
Added snap=False to pcolor default args as mpl1.3 workaround
Updated license to reflect the new year
Updated cloud cover base image
Image updates where changes limited to axes positioning
Further image updates for axes postioning, pcolor snap changes
Images changed by Cartopy update
Final image changes
Example images changes
Deriving phenomena image changed following merge of #952
Matplotlib version in .travis.yml corrected to v1.3.1.