Use an updated salsa20, poly1305 from golang.org/x/crypto

diff --git a/src/golang.org/x/crypto b/src/golang.org/x/crypto
index 0e31b188fd38db611d4fbab7de9373a95f36aae5..b2fa06b6af4b7c9bfeb8569ab7b17f04550717bf 160000 (submodule)
--- a/src/golang.org/x/crypto
+++ b/src/golang.org/x/crypto
@@ -1 +1 @@
-Subproject commit 0e31b188fd38db611d4fbab7de9373a95f36aae5
+Subproject commit b2fa06b6af4b7c9bfeb8569ab7b17f04550717bf