Bump google.golang.org/grpc from 1.15.0 to 1.56.3

Bumps [google.golang.org/grpc](https://github.com/grpc/grpc-go) from 1.15.0 to 1.56.3.
- [Release notes](https://github.com/grpc/grpc-go/releases)
- [Commits](grpc/grpc-go@v1.15.0...v1.56.3)

---
updated-dependencies:
- dependency-name: google.golang.org/grpc
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

go.mod

@@ -3,23 +3,15 @@ module github.com/fujiwara/stretcher
 go 1.12
 
 require (
-	cloud.google.com/go v0.28.0
+	cloud.google.com/go/storage v1.29.0
 	github.com/aws/aws-sdk-go v1.44.285
 	github.com/dustin/go-humanize v1.0.0
 	github.com/fujiwara/shapeio v0.0.0-20170602072123-c073257dd745
-	github.com/google/go-cmp v0.2.0 // indirect
 	github.com/google/go-github v17.0.0+incompatible // indirect
 	github.com/google/go-querystring v1.0.0 // indirect
-	github.com/google/martian v2.1.0+incompatible // indirect
-	github.com/googleapis/gax-go v2.0.0+incompatible // indirect
 	github.com/hashicorp/go-version v1.0.0 // indirect
 	github.com/tcnksm/go-latest v0.0.0-20170313132115-e3007ae9052e
-	go.opencensus.io v0.17.0 // indirect
 	golang.org/x/net v0.11.0 // indirect
-	golang.org/x/time v0.0.0-20180412165947-fbb02b2291d2 // indirect
-	google.golang.org/api v0.0.0-20180929000454-5da02d31af7d // indirect
-	google.golang.org/appengine v1.2.0 // indirect
-	google.golang.org/genproto v0.0.0-20180928223349-c7e5094acea1 // indirect
-	google.golang.org/grpc v1.15.0 // indirect
+	google.golang.org/grpc v1.56.3 // indirect
 	gopkg.in/yaml.v2 v2.2.8
 )