Fix infinite loop on CIs after Numba v0.5.0

[santisoler]

Set NUMBA_THREADING_LAYER to workqueue on Travis CIs running Ubuntu.
Solution obtained from numba/numba#5983.

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

@leouieda Using NUMBA_THREADING_LAYER=workqueue fixed the problem on Travis CI.
But, the CIs will still fail building the docs because of #174. Do you agree to merge this PR even CIs are failing?
I will then update #173, where hopefully CIs will work flawlessly.

[leouieda]

@santisoler yeah, I'm fine with merging this in 👍🏽

[santisoler]

Awesome! Now I can put CIs back on the rails!