Run tox instead setup.py test with Travis-CI. (#723)

diff --git a/.travis.yml b/.travis.yml
index fdeb58d..ce811e5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,8 @@
+sudo: false
 language: python
 python:
   - "3.6"
   - "3.7"
   - "3.8"
-install:
-  python setup.py install
-script:
-  python setup.py test
+install: pip install tox-travis
+script: tox