ci: build failure

The module CI build is currently failing https://github.com/inveniosoftware/invenio-sequencegenerator/pull/28/checks?check_run_id=1531067426#step:7:73

```
db =

def test_exceptions(db):
"""Test exceptions."""
...
with pytest.raises(InvalidResetCall) as exc:
Sequence('P1').counter.reset()
> assert 'Cannot reset sequence: children exist' in str(exc)
E AssertionError: assert 'Cannot reset sequence: children exist' in ''
E + where '' = str()

```