add file vendor/.nosearch

This prevents that e.g. `normal-top-level-add-subdirs-to-load-path`
adds "vendor/" to the `load-path`.

If you don't want to merge this following a rational similar to this: https://github.com/cask/epl/pull/21#issuecomment-37167730 then that's fine with me too. I just thought I would ask.