MAINT: update iris to use latest cartopy

Changes include fixing the transform points tests, experimental
regridding and the travis cartopy commit ref (v0.8.0).  iris pole
rotate and unrotate indexing has also been fixed to comply with
cartopy indexing of transform points.

Behaviour change includes plots now being limited by projection limits
rather than the data limits (where smaller than data limits).
Merge pull request #494 from cpelley/against_cartopy_master

updating iris to use cartopy v0.8.0