Allow installing recent virtualenv version (now that pypy supports Python 3.5)

Author: touilleMan

.travis.yml

@@ -33,7 +33,7 @@ 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 pip install virtualenv
 - travis_retry tox -e $(echo py$TRAVIS_PYTHON_VERSION-me$MONGOENGINE | tr -d . | sed -e 's/pypypy/pypy/') -- -e test
 
 # Cache dependencies installed via pip