Set custom payload version to current

Set the custom payload version to `DockerAppCustomVersionCurrent` when
creating a new bundle so as to reduce the number of required changes
when developing a new version of the custom payload.

Signed-off-by: Nick Adcock <nick.adcock@docker.com>

internal/packager/cnab.go

@@ -172,7 +172,7 @@ func ToCNAB(app *types.App, invocationImageName string) (*bundle.Bundle, error)
 		SchemaVersion: CNABVersion1_0_0,
 		Custom: map[string]interface{}{
 			internal.CustomDockerAppName: DockerAppCustom{
-				Version: DockerAppCustomVersion1_0_0,
+				Version: DockerAppCustomVersionCurrent,
 				Payload: payload,
 			},
 		},