Fix pandas array order for numpy 1.7

NumPy 1.7 preserves C/Fortran-order when copying an array, but 1.6 always creates copies as C-order.

This change forces C-order when taking a copy, thus preserving the existing behaviour.

Fixes:
 - iris.tests.test_pandas.TestDataFrameAsCube
  - test_data_frame_datetime_gregorian
  - test_data_frame_masked
  - test_data_frame_netcdftime_360
  - test_data_frame_nonotonic
  - test_data_frame_simple