Fix XRImage producing read-only data arrays and switch to pytest

[djhoese]

I know this is a lot for one PR but the bug that I'm actually fixing is only an issue with the dev version of xarray so I needed to rework the travis config anyway. So I used the opportunity to update tests to use pytest.

This "bug" is due to a recent change in xarray's expand_dims producing read-only arrays: pydata/xarray#3813

This is showing up in pytroll/satpy#1095 for one of the enhancement tests in Satpy.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[coveralls]

Coverage increased (+0.4%) to 90.619% when pulling 34e2445 on djhoese:bugfix-expand-dims-readonly into 103b269 on pytroll:master.

[mraspaud]

Looks good, just one inline question.
Also, should we add this classifier to setup.py: Programming Language :: Python :: 3 :: Only ?

[djhoese]

I don't think the classifier is needed.