configure pytest to always create coverage report

https://pytest-cov.readthedocs.io/en/latest/config.html
https://doc.pytest.org/en/latest/customize.html

.travis.yml

@@ -40,4 +40,4 @@ install:
 
 script:
 - pipenv run pylint freesurfer_volume_reader tests/*
-- pipenv run pytest --cov=freesurfer_volume_reader --cov-report=term-missing
+- pipenv run pytest

pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+addopts = --cov=freesurfer_volume_reader --cov-report=term-missing