Update GWT to 2.6.0
Update GWT to 2.6.0

To match the version used in core Gerrit.

Change-Id: I7deb6829e574d8f025613d0a815774b607077da9

GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser,GerritCodeReview/plugins_serviceuser