update install command of latest readme

README.md

@@ -27,9 +27,9 @@ pip install paddlepaddle
 # Linux GPU cuda9cudnn7
 pip install paddlepaddle-gpu
 # Linux GPU cuda8cudnn7
-pip install paddlepaddle-gpu==0.14.0.post87
+pip install paddlepaddle-gpu==0.15.0.post87
 # Linux GPU cuda8cudnn5
-pip install paddlepaddle-gpu==0.14.0.post85
+pip install paddlepaddle-gpu==0.15.0.post85
 
 # For installation on other platform, refer to http://paddlepaddle.org/
 ```