Fix missing ray import

Signed-off-by: Jonathan Shi <jhshi@ponder.io>

Author: noloerino

modin/core/execution/ray/implementations/pandas_on_ray/dataframe/dataframe.py

@@ -13,6 +13,8 @@
 
 """Module houses class that implements ``PandasDataframe`` using Ray."""
 
+import ray
+
 from ..partitioning.partition_manager import PandasOnRayDataframePartitionManager
 from modin.core.dataframe.pandas.dataframe.dataframe import PandasDataframe