Don't bind to process.exit for each spawn

Otherwise Node will issue a warning for "possible EventEmitter memory leak detected", if `selenium` is called repeatedly.

Example:

```
(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at process.EventEmitter.addListener (events.js:160:15)
    at process.on.process.addListener (node.js:773:26)
    at listenAndKill (/Users/u796/Projects/alf/node_modules/selenium-standalone/index.js:28:13)
    at Array.forEach (native)
    at standalone (/Users/u796/Projects/alf/node_modules/selenium-standalone/index.js:27:33)
```