change path property to binary to match options property in node-phantom

This PR addresses issue https://github.com/segmentio/nightmare/issues/92 and PR https://github.com/segmentio/nightmare/pull/94.

@kevva @reinpk 

I agree that it would be nice to specify the phantom binary path rather than globally installing Phantom via Homebrew or some other method.  @kevva I tested out your solution and believe I found a problem in that the `path` property in the `options` should actually be `binary`, which corresponds to the same property in the node-phantom module.  

Therefore, you would

`var phantomPath = require('phantomjs').path;`

and instantiate nightmare the same way

`new Nightmare({phantomPath: phantomPath})`

@kevva if I use your solution I will continuously get the "phantomjs-node: You don't have 'phantomjs' installed" error message from node-phantom.