Travis hook to verifies new directories in examples.json

I noticed that several menu examples in this repo are not showing up on MDN, probably because they are not listed in examples.json. I just looked at all top directories in the repository, and found that the following are not listed in examples.json:

• https://github.com/mdn/webextensions-examples/tree/master/menu-accesskey-visible
• https://github.com/mdn/webextensions-examples/tree/master/menu-remove-element
• https://github.com/mdn/webextensions-examples/tree/master/menu-search
• https://github.com/mdn/webextensions-examples/tree/master/root-cert-stats

They should be listed, and to prevent this from happening again, there should probably a travis hook that verifies whether new directories are part of examples.json.