Remove phantomenv dependency, use Homebrew to install.

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. Yes, you can overwrite the phantomenv source repository in your Boxen config, but that's not an acceptable design decision IMHO.

Also removed OS checks (Darwin vs. Linux) since they don't appear to be actually used/useful.