Update README.md and README_cn.md to latest version 1.6.1, test=develop (#21119)

README.md

@@ -26,7 +26,7 @@ pip install paddlepaddle
 # Linux GPU cuda10cudnn7
 pip install paddlepaddle-gpu
 # Linux GPU cuda9cudnn7
-pip install paddlepaddle-gpu==1.6.0.post97
+pip install paddlepaddle-gpu==1.6.1.post97
 
 
 # For installation on other platform, refer to http://paddlepaddle.org/

README_cn.md

@@ -24,7 +24,7 @@ pip install paddlepaddle
 # Linux GPU cuda10cudnn7
 pip install paddlepaddle-gpu
 # Linux GPU cuda9cudnn7
-pip install paddlepaddle-gpu==1.6.0.post97
+pip install paddlepaddle-gpu==1.6.1.post97
 
 
 # 其他平台上的安装指引请参考 http://paddlepaddle.org/