test(travis config): run pytest on test modules one by one, see #58

Details here #58

.travis.yml

@@ -7,6 +7,6 @@ install:
   - pip install --upgrade pytest-cov pytest
   - pip install codecov
 script:
-  - pytest --cov-report term --cov=neurodiffeq/
+  - for file in neurodiffeq/tests/test*py; do pytest --cov-report term --cov=$file; done
 after_success:
-  - codecov
+  - codecov