Update target branch of release script

As part of elastic/package-storage#86 the way packages are released is changed. The release process will be snapshot -> staging -> production. This PR now first switches over to directly open PR's against production to move away from master. This allows us to update the deployment of epr.elastic.co to point to production branch instead and start to cleanup / remove the master branch.

The second step will be to adjust the script that it directly pushed to snapshot and we then have a release script to promote packages from snapshot to staging to production.

dev/update-package-storage/git.go

@@ -24,14 +24,14 @@ func checkoutMasterBranch(err error, options updateOptions) error {
 	if err != nil {
 		return err
 	}
-	return runGitCommand(options, "checkout", "master")
+	return runGitCommand(options, "checkout", "production")
 }
 
 func rebaseUpstreamMaster(err error, options updateOptions) error {
 	if err != nil {
 		return err
 	}
-	return runGitCommand(options, "rebase", "upstream/master")
+	return runGitCommand(options, "rebase", "upstream/production")
 }
 
 func addToIndex(err error, options updateOptions, packageName, packageVersion string) error {