Compile error

Environment:
go version go1.13 linux/amd64
libprotoc 3.14.0

Here follows my operation step:
1. git clone https://github.com/googleapis/gnostic
2. cd gnostic
3. make

And then errors happened:
```
# github.com/googleapis/gnostic/metrics/vocabulary
metrics/vocabulary/vocabulary.go:71:29: cannot use v (type *gnostic_metrics_v1.Vocabulary) as type protoreflect.ProtoMessage in argument to "google.golang.org/protobuf/proto".Marshal:
*gnostic_metrics_v1.Vocabulary does not implement protoreflect.ProtoMessage (missing ProtoReflect method)
metrics/vocabulary/vocabulary.go:86:29: cannot use v (type *gnostic_metrics_v1.VocabularyList) as type protoreflect.ProtoMessage in argument to "google.golang.org/protobuf/proto".Marshal:
*gnostic_metrics_v1.VocabularyList does not implement protoreflect.ProtoMessage (missing ProtoReflect method)
metrics/vocabulary/vocabulary.go:101:29: cannot use v (type *gnostic_metrics_v1.VersionHistory) as type protoreflect.ProtoMessage in argument to "google.golang.org/protobuf/proto".Marshal:
*gnostic_metrics_v1.VersionHistory does not implement protoreflect.ProtoMessage (missing ProtoReflect method)
metrics/vocabulary/vocabulary.go:212:23: cannot use v (type *gnostic_metrics_v1.Vocabulary) as type protoreflect.ProtoMessage in argument to "google.golang.org/protobuf/proto".Unmarshal:
*gnostic_metrics_v1.Vocabulary does not implement protoreflect.ProtoMessage (missing ProtoReflect method)
make: *** [all] Error 2
```