Upgrade bazlets to latest stable-2.15 to build with 2.15.5 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.5 API

Change-Id: I47f089d3e3bd36a3815b0ae5f353532ac890bbb6

GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers,GerritCodeReview/plugins_reviewers