Specify virtualenv>=15.2.0
It appears 15.0.2 generates a bad reference to the pip._install module.
I've tested that 15.2.0 does not so, although I don't really understand
why and when this changed (my current guess is the Travis base image
changed), this should fix pulumi/pulumi#1103.
@@ -13,7 +13,7 @@ GOMETALINTER := ${GOMETALINTERBIN} --config=../../Gometalinter.json
include ../../build/common.mk
ensure::
- $(PIP) install virtualenv
+ $(PIP) install 'virtualenv>=15.2.0'
rm -rf $(PYENV) && virtualenv --python=$(PYTHON) $(PYENV)/
echo "export PYTHONPATH=$${PYTHONPATH}:$(shell pwd)/$(PYENVLIB)/python2.7/site-packages" >> $(PYENVBIN)/activate
( \
