use phantomjs path if it's not in PATH

The `jasmine` gem has a dependency on the `phantomjs` gem. The latter is cool because if `phantomjs` is not installed into `PATH`, it installs it into `~/.phantomjs` and uses it from there. It does not, however, alter `PATH`.

The `phantomjs` gem only does the above if the `Phantomjs.path` method is called, and then it returns the actual path to Phantom, regardless of where it resides.

All I did in this PR was change the hard-coded `phantomjs` executable to use `Phantomjs.path`, which will install it as needed and use it even if it was installed outside of `PATH`.