Release 1.3.0
Bump version to v1.3.1-dev
Merge pull request #426 from pp-mo/v1x3_release

V1x3 release
Force mpl v1.2.0 for Travis-CI.
Merged v1.3.x branch into master.
Helper function for regridding that returns x and y coords.
Review actions. Moved tests and improved docstring.
Merge pull request #437 from esc24/regrid_helper

Helper function for regridding that returns x and y coords.
Travis-CI non git clone of Cartopy.
Merge pull request #436 from bjlittle/travis_no_clone_cartopy

Travis-CI non git clone of Cartopy.
Add an optimisation for single valued coordinate constraints.