Able to run venus run command with PhantomJS

This pull request is in reference to the following issue: https://github.com/linkedin/venus.js/issues/290

When defining commands and command line options with commander (https://npmjs.org/package/commander), it seems like the default case that was added to support running tests without the "run" command seemed to trump all other commands defined in Venus.js. 

I went ahead and defined the command for the default case in it's own if clause that would only be executed if no commands are defined (e.g., "init", "demo", "run").

With the change, I was able to run the following commands successfully:

```javascript
venus init
venus demo
venus run -t examples/mocha/Greeter/specs/Greeter.spec.js
venus run -t examples/mocha/Greeter/specs/Greeter.spec.js -n
venus examples/mocha/Greeter/specs/Greeter.spec.js
venus examples/mocha/Greeter/specs/Greeter.spec.js -n
```