Update go.mod and go.sum. (#113)

* Update go.mod and go.sum.

* Upgrade ocagent exporter version.

go.mod

@@ -1,19 +1,18 @@
 module contrib.go.opencensus.io/exporter/stackdriver
 
 require (
-	cloud.google.com/go v0.36.0
-	contrib.go.opencensus.io/exporter/ocagent v0.4.8
+	cloud.google.com/go v0.37.1
+	contrib.go.opencensus.io/exporter/ocagent v0.4.9
 	contrib.go.opencensus.io/resource v0.0.0-20190131005048-21591786a5e0
-	github.com/aws/aws-sdk-go v1.17.5
+	github.com/aws/aws-sdk-go v1.18.6
 	github.com/census-instrumentation/opencensus-proto v0.2.0
-	github.com/golang/protobuf v1.3.0
+	github.com/golang/protobuf v1.3.1
 	github.com/google/go-cmp v0.2.0
 	github.com/grpc-ecosystem/grpc-gateway v1.8.5 // indirect
-	go.opencensus.io v0.19.2-0.20190319182201-ebb7978abd3f // TODO(songy23): use the released versoin once new release is publised.
-	golang.org/x/net v0.0.0-20190311183353-d8887717615a
-	golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421
+	go.opencensus.io v0.19.1
+	golang.org/x/net v0.0.0-20190320064053-1272bf9dcd53
+	golang.org/x/oauth2 v0.0.0-20190319182350-c85d3e98c914
 	google.golang.org/api v0.2.0
 	google.golang.org/genproto v0.0.0-20190307195333-5fe7a883aa19
-	google.golang.org/grpc v1.19.0
-	gopkg.in/yaml.v2 v2.2.2 // indirect
+	google.golang.org/grpc v1.19.1
 )