Phantomjs: Yosemite fixes

This is a pretty clunky workaround, and I’m open to suggestions on how to make this more automatic for users, but it does the job if the maintainers are happy with it.
@tdsmith It was originally inside ``` install ```, but PhantomJS is kinda huge and having people download the entire repository only to be told "Nope, go do HEAD instead" seemed a bit of a pain in the behind for users. But I'll talk out the ``` Requirement ``` idea with Mike and see if we can go down that considerably better path instead.