change google.golang.org/api dependency to v0.3.1 (#1096)

step 12/15.
diff --git a/go.mod b/go.mod
index 7f6e451..cc9febc 100644
--- a/go.mod
+++ b/go.mod
@@ -8,6 +8,6 @@
 	github.com/openzipkin/zipkin-go v0.1.6
 	github.com/prometheus/client_golang v0.9.3-0.20190127221311-3c4408c8b829
 	golang.org/x/net v0.0.0-20190311183353-d8887717615a
-	google.golang.org/api v0.3.1-alpha
+	google.golang.org/api v0.3.1
 	google.golang.org/grpc v1.19.0
 )