Normalise array order after curl.

The NumPy 1.6 vs. 1.7 change to np.append behaviour rears its head once again!

This change forces the affected test result to have C-order.

Fixes:
 - iris.tests.test_analysis_calculus.TestCalculusWKnownSolutions
  - test_contrived_non_spherical_curl2

NB. This is the last change for NumPy 1.7! \o/