travis-ci: upgrade pipenv to fix failing `pipenv sync` with python3.4

$ pipenv sync --dev
[...]
  File "/home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 704, in from_line
    line, extras = _strip_extras(line)
TypeError: 'module' object is not callable
https://travis-ci.org/fphammerle/free-disk/jobs/525329085#L225

https://github.com/pypa/pipenv/issues/2924#issuecomment-429573525

.travis.yml

@@ -12,7 +12,7 @@ python:
 dist: xenial
 
 install:
-- pip install pipenv
+- pip install --upgrade pipenv>=2018.10.9
 - pipenv sync --dev
 - pipenv graph