Added clear cache tip to sandbox.gpuarray placeholder

theano/sandbox/gpuarray/__init__.py

@@ -4,6 +4,7 @@ which refered to theano.sandbox.gpuarray."""
 import warnings
 from theano.gpuarray import *
 
-message = "theano.sandbox.gpuarray has been moved to theano.gpuarray." + \
-    " Please update your code and pickles."
+message = ("theano.sandbox.gpuarray has been moved to theano.gpuarray. "
+    "Please update your code and pickles. If the warning persists, "
+    "clear theano's cache ('$theano/bin/theano-cache clear').")
 warnings.warn(message)