Switch to using the dep tool and update all the dependencies

vendor/google.golang.org/grpc/reflection/grpc_testing/proto2.proto

@@ -0,0 +1,8 @@
+syntax = "proto2";
+
+package grpc.testing;
+
+message ToBeExtended {
+  required int32 foo = 1;
+  extensions 10 to 30;
+}