MAINT: remove NumPy <1.6 compatibility

To be merged only after a decision has been made wrt. the minimum NumPy version required.
This is the bit that is currently failing a test. I must admit I don't really understand this, so I can just revert and leave it for some other time. The `_aligned_zeros` docstring suggests that it actually works around a NumPy 1.9.1 bug...