Update bazlets to latest stable-2.15 to build with 2.15.4 API
Update bazlets to latest stable-2.15 to build with 2.15.4 API

Change-Id: I96568a456d991f9e847b8eaf87a0cc161f06d85e

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