Automatically download new ciphers

I had an idea so that we don't have to push a new gem version every time a new cipher is added. I can see that getting old very fast. :) Basically with this commit the Youtube plugin will do a head request on the ciphers.yml file in the repository. If it detects that the file has changed (by using the Content-Length header) it will download the file and update the local version. This way we can just add new ciphers to the ciphers.yml file and they will be available right away. The cost is that each time the Youtube plugin runs it makes a head request to the file, but that takes less than a second so I don't think that's a big issue.