Fix travis-ci test failure

[cdeil]

Currently the master branch is broken on travis-ci.
In this PR I try to understand and fix the test errors.

[cdeil]

@astrofrog @mdboom I think this might be an Astropy or pytest issue:
https://travis-ci.org/gammapy/gammapy/jobs/26885100#L808

The TestGammaSpectralCube.test_flux_array test should be skipped because of

@pytest.mark.skipif('not HAS_SCIPY')
class TestGammaSpectralCube(object):

(see here) and because HAS_SCIPY = False (see here).

I can't reproduce this locally, so I think it must be a problem with a certain Astropy or pytest version.

Do you understand what is going on or can you reproduce?