TST: fix sparse test errors due to axis=-1,-2 usage in np.matrix.sum().

Negative axis arguments are not supported by numpy < 1.7.0.
Fixes part of gh-3330.
Merge pull request #3391 from rgommers/sparse-errors

TST: sparse: fix sparse test errors due to axis=-1,-2 usage in np.matrix.sum().
BUG: sparse/compressed: work around bug in np.unique in earlier numpy versions
Merge pull request #3393 from pv/fix-sparse-unique

BUG: sparse/compressed: work around bug in np.unique in earlier numpy versions