Require xgboost_ray>=0.1.8

Author: Yard1

setup.py

@@ -9,4 +9,4 @@
     long_description="A distributed backend for LightGBM built on top of "
     "distributed computing framework Ray.",
     url="https://github.com/ray-project/lightgbm_ray",
-    install_requires=["lightgbm>=3.2.1", "xgboost_ray>=0.1.6"])
+    install_requires=["lightgbm>=3.2.1", "xgboost_ray>=0.1.8"])