Fix version check in push-to-trunk.

The local master branch is not kept up-to-date by the script.

BUG=
TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/411593003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22546 ce2b1a6d-e550-0410-aec6-3dcde31c8c00