TST: Fix ValueError: Buffer dtype mismatch, expected 'npy_intp' but got 'long' on win-amd64

This fixes the following test error on win-amd64. See also issue #3008.
```
======================================================================
ERROR: sklearn.utils.tests.test_sparsefuncs.test_densify_rows
----------------------------------------------------------------------
Traceback (most recent call last):
  File "X:\Python27-x64\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "X:\Python27-x64\lib\site-packages\sklearn\utils\tests\test_sparsefuncs.py", line 43, in test_densify_rows
    assign_rows_csr(X, rows, np.arange(out.shape[0], dtype=np.intp)[::-1], out)
  File "sparsefuncs_fast.pyx", line 313, in sklearn.utils.sparsefuncs_fast.assign_rows_csr (sklearn\utils\sparsefuncs_fast.c:4235)
ValueError: Buffer dtype mismatch, expected 'npy_intp' but got 'long'

```