Update .gitignore with test/test262-es6/data.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/646933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24515 ce2b1a6d-e550-0410-aec6-3dcde31c8c00