Travis: don't run with uninstallable NumPy versions

.. rather than permitting failure and wasting resources on testing it anyway

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Travis: list the installed packages the current test is ran with

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Travis: remove NumPy if necessary

... ignoring failures with the assumption that failing to remove is
always caused by NumPy not being present in the first place.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Travis: test with NumPy 1.8 as well

.. but permit failures as (apparently), for now, we're failing with
NumPy 1.8.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Unittest: expect two quantities with the same value no to be unequal

I.e. currently this goes wrong, because __ne__ isn't implemented. And
according to the Python docs for object.__eq__: "when defining __eq__(),
one should also define __ne__() so that the operators will behave as
expected.".

Signed-off-by: Giel van Schijndel <giel.van.schijndel@asml.com>
Implement Quantity.__ne__

This ensures that given two Quantities x and y (x == y) == not (x != y).

Signed-off-by: Giel van Schijndel <giel.van.schijndel@asml.com>
Merge branch 'travis-updates' into fix-inequality-comparision

Provides Travis CI config changes to prevent false negatives in test
output.