go.tools/dashboard/app: require version=2 for commit updates
go.tools/dashboard/app: require version=2 for commit updates

This will prevent older, buggy builders from posting commits.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/153930043

bryanxu/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools