Upgrade tox, virtualenv, py, and pluggy on Travis CI

.travis.yml

@@ -18,10 +18,10 @@ matrix:
       sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
 
 install:
-  - pip install tox==2.9.1
-  - pip install virtualenv==15.2.0
-  - pip install py==1.4.30
-  - pip install pluggy==0.5.2
+  - pip install tox==3.13.2
+  - pip install virtualenv==16.7.4
+  - pip install py==1.8.0
+  - pip install pluggy==0.12.0
 
 before_script:
   # Run flake8 tests only on Python 2.7 and 3.7...