Migrate away from phantomenv, use Homebrew for phantomjs installs

This is a PR to install phantomjs through Homebrew alone, without the need of phantomenv.

As mentioned in https://github.com/boxen/puppet-phantomjs/issues/17 https://github.com/wfarr/phantomenv suffers from lack of active development and hasn't been able to download recent releases of phanomjs (1.9.7, 1.9.8) for sometime.

Also removed OS checks (Darwin vs. Linux) since they don't appear to be actually used/useful.