Update gRPC to v1.7.2

This solves a problem where Gitaly connections through Unix sockets
would not work when a proxy was configures, even if a no_proxy setting
was specified

Gemfile.lock

@@ -379,10 +379,10 @@ GEM
       rake
     grape_logging (1.7.0)
       grape
-    grpc (1.6.6)
+    grpc (1.7.2)
       google-protobuf (~> 3.1)
       googleapis-common-protos-types (~> 1.0.0)
-      googleauth (~> 0.5.1)
+      googleauth (>= 0.5.1, < 0.7)
     gssapi (1.2.0)
       ffi (>= 1.0.1)
     haml (4.0.7)