add virtualenv<14.0.0 so that we can continue supporting pypy3

Author: wojcikstefan

.travis.yml

@@ -31,6 +31,7 @@ install:
 - travis_retry pip install coveralls
 - travis_retry pip install flake8
 - travis_retry pip install tox>=1.9
+- travis_retry pip install "virtualenv<14.0.0"  # virtualenv>=14.0.0 has dropped Python 3.2 support (and pypy3 is based on py32)
 - travis_retry tox -e $(echo py$TRAVIS_PYTHON_VERSION-me$MONGOENGINE | tr -d . | sed -e 's/pypypy/pypy/') -- -e test
 
 before_script: