Update supported PyTorch versions in README

Author: ajaykarpur

README.rst

@@ -226,7 +226,7 @@ PyTorch SageMaker Estimators
 
 With PyTorch SageMaker Estimators, you can train and host PyTorch models on Amazon SageMaker.
 
-Supported versions of PyTorch: ``0.4.0``, ``1.0.0``, ``1.1.0``, ``1.2.0``.
+Supported versions of PyTorch: ``0.4.0``, ``1.0.0``, ``1.1.0``, ``1.2.0``, ``1.3.1``.
 
 We recommend that you use the latest supported version, because that's where we focus most of our development efforts.