Peg versions of test tools

Author: Jordan Adler

.travis.yml

@@ -22,7 +22,10 @@ matrix:
       sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
 
 install:
-  - pip install tox
+  - pip install tox==2.9.1
+  - pip install virtualenv==15.2.0
+  - pip install py==1.4.30
+  - pip install pluggy==0.5.2
 
 before_script:
   # Run flake8 tests only on Python 2.7 and 3.7...

tox.ini

@@ -2,5 +2,7 @@
 envlist = py26,py27,py33,py34,py35,py36,py37
 
 [testenv]
-deps = pytest
+deps =
+    pytest
+    unittest2
 commands = pytest {posargs}