Switched to official version of PhantomJS arhcive.

@stomita, I hope you don't mind this pull request.

I feel that some people like me would feel more security to depend on an phantomJS build from its official download location. It would also nice to use `https` for download to guard against minimal risk of dns spoofing in heroku / amazon aws.

I tested the patch and it was working with heroku.

The downside is that I am not including `fontconfig` and `freetype`.

Since you are the "official third party" buildpack for PhantomJS, even if you don't apply the patch, I wish you consider keeping this PR open for the paranoids out there. :-)

@stomita, thanks a lot for open-source your buildpack. It definitely helped me tremendously!