Remove dependence on outdated version of minimist

[hobindar]

peerjs-server currently relies on an outdated version of minimist (via optimist) that has a known security vulnerability. This dependence should be removed or otherwise updated.

Further, optimist is no longer maintained. It has not been updated since 2014.

This seems easy enough to resolve however. I believe optimist could just be swapped out for yargs.

I'll create a PR for this shortly.

[afrokick]

Thanks @hobindar Released in 0.5.3