Updating to use Phapper over Phantomjs NPM

> @macbre If you're not interested, I understand. Simply close the PR. :)

##### Why Phapper:

1. Phapper more cleanly supports using existing installations of PhantomJS, allowing for users to have a global install, saving time in installing the module.
2. Phapper supports running PhantomJS script Synchronously, which has it benefits when running a large number of processes through it. Ideally, `rum-multiple.js` would be updated to run synchronously to avoid errors due to process limitations (which I've seen when running a large number of tests).

##### Downside to Phapper (over Phantomjs NPM):

Phapper doesn't currently support PhantomJS installation on Windows. However, it should work with custom PhantomJS installations (although I don't have a Window machines accessible to verify).