Update dep: github.com/golang/protobuf v1.3.5 -> v1.5.1

Thanks to https://go-review.googlesource.com/c/protobuf/+/300869/ its
feasible now.

bill-of-materials.json

@@ -593,6 +593,15 @@
 			}
 		]
 	},
+	{
+		"project": "google.golang.org/protobuf",
+		"licenses": [
+			{
+				"type": "BSD 3-clause \"New\" or \"Revised\" License",
+				"confidence": 0.9663865546218487
+			}
+		]
+	},
 	{
 		"project": "gopkg.in/cheggaaa/pb.v1",
 		"licenses": [