Pin sklearn version (#8465)

ci/travis/install-dependencies.sh

@@ -212,7 +212,7 @@ install_dependencies() {
     pip_packages=(scipy tensorflow=="${TF_VERSION:-2.0.0b1}" cython==0.29.0 gym \
       opencv-python-headless pyyaml pandas==0.24.2 requests feather-format lxml openpyxl xlrd \
       py-spy pytest pytest-timeout networkx tabulate aiohttp uvicorn dataclasses pygments werkzeug \
-      kubernetes flask grpcio pytest-sugar pytest-rerunfailures pytest-asyncio scikit-learn numba \
+      kubernetes flask grpcio pytest-sugar pytest-rerunfailures pytest-asyncio scikit-learn==0.22.2 numba \
       Pillow prometheus_client)
     if [ "${OSTYPE}" != msys ]; then
       # These packages aren't Windows-compatible

docker/tune_test/requirements.txt

@@ -18,6 +18,7 @@ opencv-python-headless
 pandas
 pytest-remotedata>=0.3.1
 pytest-timeout
+scikit-learn==0.22.2
 scikit-optimize
 sigopt
 smart_open