Update oc version in go.mod. (#129)

Author: songy23

go.mod

@@ -1,17 +1,17 @@
 module contrib.go.opencensus.io/exporter/stackdriver
 
 require (
-	cloud.google.com/go v0.37.2
-	contrib.go.opencensus.io/exporter/ocagent v0.4.11
+	cloud.google.com/go v0.37.4
+	contrib.go.opencensus.io/exporter/ocagent v0.4.12
 	contrib.go.opencensus.io/resource v0.0.0-20190131005048-21591786a5e0
 	github.com/aws/aws-sdk-go v1.18.6
 	github.com/census-instrumentation/opencensus-proto v0.2.0
 	github.com/golang/protobuf v1.3.1
 	github.com/google/go-cmp v0.2.0
-	go.opencensus.io v0.20.0
+	go.opencensus.io v0.20.2
 	golang.org/x/net v0.0.0-20190320064053-1272bf9dcd53
 	golang.org/x/oauth2 v0.0.0-20190319182350-c85d3e98c914
-	google.golang.org/api v0.3.0
-	google.golang.org/genproto v0.0.0-20190307195333-5fe7a883aa19
+	google.golang.org/api v0.3.2
+	google.golang.org/genproto v0.0.0-20190404172233-64821d5d2107
 	google.golang.org/grpc v1.19.1
 )