fix confused message when phantomjs is not exists

@lepture, in this patch, I added phantomjs as dependency because i got a confusing message when i have no phantomjs installed globally:
```
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:1001:11)
    at Process.ChildProcess._handle.onexit (child_process.js:792:34)
```