Update GWT to 2.6.1
Update GWT to 2.6.1

To match the version used in core Gerrit.

Change-Id: If6ffbf8762f0c88cc74820fc62192796ed1ddd86

GerritCodeReview/plugins_cookbook-plugin,GerritCodeReview/plugins_cookbook-plugin,GerritCodeReview/plugins_cookbook-plugin,akilman/appd-hackathon-2016,akilman/appd-hackathon-2016,pgroudas/gerrit-keep-approvals,pgroudas/gerrit-keep-approvals,akilman/appd-hackathon-2016