Upgrade bazlets to latest stable-2.15 to build with 2.15.6 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.6 API

Change-Id: I480cfe40d45043c7e55a0ce7cf397d5bb1b4feaa

GerritCodeReview/plugins_quota,GerritCodeReview/plugins_quota,GerritCodeReview/plugins_quota