Add application/x-javascript to nginx.conf

Some instances of Nginx server JS with the mime-type `application/x-javascript` and thus the `nginx.conf` server needs to be told to watch for it.

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For whatever reason, on my server (out of the box Nginx installed on a fresh CentOS), my JS is coming through with `application/x-javascript`, so unless I add that to the `nginx.conf`, my JavaScript doesn't get gzipped (which is sorta of a big deal when my site's mostly JavaScript!).