try even older version

.travis.yml

@@ -63,7 +63,7 @@ install:
   - conda install nlopt petsc4py petsc $MUMPS mpi4py scipy $MPI
   - pip install flake8       # Conda installs downgrade pytest on python3.4
   - pip install DFO-LS
-  - pip install coverage==5.0
+  - pip install coverage==4.5.4
   - pip install pytest
   - pip install pytest-cov
   - pip install pytest-timeout