Upgrade bazlets to latest stable-2.15 to build with 2.15.13 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.13 API

Change-Id: Iefafb578f5a7b7b7cea34cb8ab55da2f15dd0c80

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