setMaxListeners(0) on reverseProxy to squelch warnings

reverseProxy can have many subscribers to the 'drain' event when large
uploads occur. Without this fix I get the following warning printed in the log:

```
(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at ClientRequest.EventEmitter.addListener (events.js:160:15)
    at ClientRequest.EventEmitter.once (events.js:179:8)
    at IncomingMessage.<anonymous> (/Users/jcheng/shiny-server-2/node_modules/http-proxy/lib/node-http-proxy/http-proxy.js:390:22)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at Object.resume (/Users/jcheng/shiny-server-2/node_modules/http-proxy/lib/node-http-proxy.js:253:18)
    at HttpProxy.proxyRequest (/Users/jcheng/shiny-server-2/node_modules/http-proxy/lib/node-http-proxy/http-proxy.js:428:16)
    at /Users/jcheng/shiny-server-2/lib/proxy/http.js:142:31
    at _fulfilled (/Users/jcheng/shiny-server-2/node_modules/q/q.js:830:54)
    at resolvedValue.promiseSend.done (/Users/jcheng/shiny-server-2/node_modules/q/q.js:860:30)
    at makePromise.promise.promiseSend (/Users/jcheng/shiny-server-2/node_modules/q/q.js:487:13)
```