Change assert imports

[mblondel]

lightning was relying on sklearn.utils.testing to import the assert_* functions but they got removed from sklearn. We should import them from numpy.testing for the assert_array_* and nose.tools for the others. @StrikerRUS do you want to tackle this?

[mblondel]

We could import all the assertion functions in impl/tests/utils.py. This way we can do from lightning.impl.tests.utils import assert_array_equal etc (I never remember which assertions are defined numpy.testing and which assertions are defined in nose.tools)

[StrikerRUS]

OK, I can take this.