Use an updated github.com/agl/ed25519

diff --git a/src/github.com/agl/ed25519 b/src/github.com/agl/ed25519
index 278e1ec8e8a6e017cd07577924d6766039146ced..5312a61534124124185d41f09206b9fef1d88403 160000 (submodule)
--- a/src/github.com/agl/ed25519
+++ b/src/github.com/agl/ed25519
@@ -1 +1 @@
-Subproject commit 278e1ec8e8a6e017cd07577924d6766039146ced
+Subproject commit 5312a61534124124185d41f09206b9fef1d88403