go.mod: update dependencies for grpc-gateway to rid protoc-gen-go/* (#83

)

Updates the grpc-gateway, whose prior transitive dependency
for "github.com/golang/protobuf/protoc-gen-go/generator"
produced a deprecation warning.

After this change, verified with `go mod why`
```shell
$ go mod why "github.com/golang/protobuf/protoc-gen-go/generator"
(main module does not need package github.com/golang/protobuf/protoc-gen-go/generator)
```

Fixes #82

go.mod

@@ -2,15 +2,15 @@ module contrib.go.opencensus.io/exporter/ocagent
 
 require (
 	github.com/census-instrumentation/opencensus-proto v0.2.1 // this is to match the version used in census-instrumentation/opencensus-service
-	github.com/golang/protobuf v1.3.2
-	github.com/google/go-cmp v0.3.0
-	github.com/grpc-ecosystem/grpc-gateway v1.9.4 // indirect
-	go.opencensus.io v0.22.0
-	golang.org/x/net v0.0.0-20190628185345-da137c7871d7 // indirect
-	golang.org/x/sys v0.0.0-20190712062909-fae7ac547cb7 // indirect
-	google.golang.org/api v0.7.0
-	google.golang.org/genproto v0.0.0-20190716160619-c506a9f90610 // indirect
-	google.golang.org/grpc v1.22.0
+	github.com/golang/protobuf v1.4.2
+	github.com/google/go-cmp v0.4.0
+	github.com/grpc-ecosystem/grpc-gateway v1.14.6 // indirect
+	go.opencensus.io v0.22.3
+	golang.org/x/net v0.0.0-20200520182314-0ba52f642ac2 // indirect
+	golang.org/x/sys v0.0.0-20200523222454-059865788121 // indirect
+	google.golang.org/api v0.25.0
+	google.golang.org/genproto v0.0.0-20200527145253-8367513e4ece // indirect
+	google.golang.org/grpc v1.29.1
 )
 
 go 1.13