[Thumbor] reply 204 on every HEAD method

Nginx reverse HTTP service does not provide (out of the box) method to check health on upstream services.
It only do periodically HEAD requests on the / given its configuration.

In thumbor HEAD method is not implemented resulting in 405 method not allowed.

I just added a basic implementation of head in handler that only reply "204 no content" on every requests. so that nginx service should be happy.