glide: pin sphinx package dependency at the commit level

With this commit we now pin the version of lightning-onion we build
against at a particular build on the Sphinx package as the package will
be modified in order to accommodate for future changes in the spec.

glide.yaml

@@ -55,7 +55,7 @@ import:
 - package: google.golang.org/grpc
   version: ^1.0.0
 - package: github.com/lightningnetwork/lightning-onion
-  version: master
+  version: a527838cac5e47260fb61ed155b9b24a6d6a10cc
 - package: github.com/grpc-ecosystem/grpc-gateway
   version: ^1.1.0
 - package: github.com/aead/chacha20