Cannot build on travis.yml pytest-cov

[gtnx]

• Date you used Cookiecutter PyPackage: 2018-09-18
• Cookiecutter version used, if any: 1.6.0
• Python version, if any: 3.6

Description

I want to add pytest-cov but travis builds fail.

What I Did

• Add pytest-cov==2.6.0 in requirements_dev.txt
• Add in tox.ini:

# [pytest]
# addopts = --cov

Then i get in travis:

message="Plugin %r could not be loaded: %s!" % (ep.name, e))
pluggy.manager.PluginValidationError: Plugin 'pytest_cov' could not be loaded: (pluggy 0.7.1 (/home/travis/build/optimdata/pynumeral/.tox/py36/lib/python3.6/site-packages), Requirement.parse('pluggy<0.7,>=0.5'), {'pytest'})!
ERROR: InvocationError for command '/home/travis/build/optimdata/pynumeral/.tox/py36/bin/py.test --basetemp=/home/travis/build/optimdata/pynumeral/.tox/py36/tmp' (exited with code 1)