Remove username from branch name as GitHub will add it automatically

when make a pull request from a forked repo.
This also satisfies @bspencer curiosity at
https://lists.crosswalk-project.org/pipermail/crosswalk-dev/2014-October/002162.html
Merge pull request #1116 from zqzhang/readme-username-remove

Remove username from branch name as GitHub will add it automatically