Upgrade bazlets to latest stable-2.15 to build with 2.15.8 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.8 API

Change-Id: Ib6bcfee2ad04f75b5eb02fc3e946462c0dc7334b

GerritCodeReview/plugins_delete-project,GerritCodeReview/plugins_delete-project,GerritCodeReview/plugins_delete-project,GerritCodeReview/plugins_delete-project