Merge branch 'hotfix/pylint'

* hotfix/pylint:
  Update travis.yml, pylint error in 3.7

.travis.yml

@@ -5,7 +5,7 @@ python:
 install:
   - pip install --upgrade pip
   - pip install -e .
-  - pip install --upgrade codecov pylint pytest pytest-cov matplotlib
+  - pip install --upgrade codecov pylint astroid pytest pytest-cov matplotlib
 script:
   - pylint pysocialforce
   - pytest --cov=pysocialforce tests/