MAINT only build versions list on master branch

Author: lesteve

build_tools/circle/build_doc.sh

@@ -116,12 +116,11 @@ pip install git+https://github.com/numpy/numpydoc
 # Build and install scikit-learn in dev mode
 python setup.py develop
 
-# TO BE UNCOMMENTED BEFORE MERGE
-###if [[ "$CIRCLE_BRANCH" =~ ^master$ && -z "$CI_PULL_REQUEST" ]]
-###then
+if [[ "$CIRCLE_BRANCH" =~ ^master$ && -z "$CI_PULL_REQUEST" ]]
+then
     # List available documentation versions if on master
     python build_tools/circle/list_versions.py > doc/versions.rst
-###fi
+fi
 
 # The pipefail is requested to propagate exit code
 set -o pipefail && cd doc && make $MAKE_TARGET 2>&1 | tee ~/log.txt