MAINT make build_doc.sh work if _VERSION env variables are not set
@@ -106,8 +106,8 @@ conda update --yes --quiet conda
# Configure the conda environment and put it in the path using the
# provided versions
-conda create -n $CONDA_ENV_NAME --yes --quiet python="$PYTHON_VERSION" numpy scipy \
- cython pytest coverage matplotlib="$MATPLOTLIB_VERSION" sphinx=1.6.2 pillow
+conda create -n $CONDA_ENV_NAME --yes --quiet python="${PYTHON_VERSION:-*}" numpy scipy \
+ cython pytest coverage matplotlib="${MATPLOTLIB_VERSION:-*}" sphinx=1.6.2 pillow
source activate testenv
pip install sphinx-gallery
