travis: set virtualenv version to restore build on py33

.travis.yml

@@ -26,8 +26,9 @@ python:
 
 install:
     # Optimisation: build requirements as wheels, which get cached by Travis
-    - pip install "pip>=9.0" wheel
-    - pip install "setuptools>=38"
+    - pip install "pip>=9.0" wheel  # pip 9.0 understands `python_requires` constraints
+    - pip install "setuptools>=38"  # setuptools >= 38 are capable of using prebuilt wheels
+    - pip install "virtualenv<16.0.0"  # virtualenv 16.0.0 drops support for py33 without properly declaring it
     - pip install tox-travis
     - pip install flake8
     # protobuf-related dependencies