Error: spawnSync ./phantomjs ENOENT

Hi tj,

Sorry for dig this out, I got `Error: spawnSync ./phantomjs ENOENT` error when I try to run capture locally via `node script.js`, (script.js is basically the index.js of capture, and I did put phantomjs and convert.js in the same directory). Could you please give me a hint where the problem might be? Thank you.

```js
const proc = require('child_process')

const args = ['convert.js', JSON.stringify({
'url': 'https://google.com',
'backgroundColor': 'white',
'format': 'jpeg'
})]
const capture = proc.execFileSync('./phantomjs', args).toString()
console.log(capture)
```