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: I694ac5e5737b642834a10b8d71b54fc9068b50da

GerritCodeReview/plugins_javamelody,GerritCodeReview/plugins_javamelody,GerritCodeReview/plugins_javamelody