all: use google.golang.org/protobuf v1.20.0

Change-Id: Ib6c3a09e41548008897bacd54aa8fc6939ea8350
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221786
Reviewed-by: Herbie Ong <herbie@google.com>

go.mod

@@ -4,5 +4,5 @@ go 1.9
 
 require (
 	github.com/google/go-cmp v0.4.0
-	google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60
+	google.golang.org/protobuf v1.20.0
 )

go.sum

@@ -1,5 +1,6 @@
 github.com/golang/protobuf v1.4.0-rc.1/go.mod h1:ceaxUfeHdC40wWswd/P6IGgMaK3YpKi5j83Wpe3EHw8=
 github.com/golang/protobuf v1.4.0-rc.1.0.20200221234624-67d41d38c208/go.mod h1:xKAWHe0F5eneWXFV3EuXVDTCmh+JuBKY0li0aMyXATA=
+github.com/golang/protobuf v1.4.0-rc.2/go.mod h1:LlEzMj4AhA7rCAGe4KMBDvJI+AwstrUpVNzEA03Pprs=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.3.1/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/google/go-cmp v0.4.0 h1:xsAVV57WRhGj6kEIi8ReJzQlHHqcBYCElAvkovg3B/4=
@@ -10,3 +11,5 @@ google.golang.org/protobuf v0.0.0-20200109180630-ec00e32a8dfd/go.mod h1:DFci5gLY
 google.golang.org/protobuf v0.0.0-20200221191635-4d8936d0db64/go.mod h1:kwYJMbMJ01Woi6D6+Kah6886xMZcty6N08ah7+eCXa0=
 google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60 h1:qkfzMNEf79BNs1//mQGZjYHIXAOv+AOdvPnMsU6R+1I=
 google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60/go.mod h1:cfTl7dwQJ+fmap5saPgwCLgHXTUD7jkjRqWcaiX5VyM=
+google.golang.org/protobuf v1.20.0 h1:SsQNHvKpk2VTiWoQ5Pqkt3Go/c2ly77C+v2Lggu5Qek=
+google.golang.org/protobuf v1.20.0/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=