Phantomjs: Yosemite Fix

It turns out we already have a MaximumMacOSRequirement floating around,
[here](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/
net-snmp.rb). I don’t know if @mikemcquaid wants me to turn this into a
regular requirement block consequently, or whether we just want to
confine these things to each formula as they crop up. (Which is fairly
rarely).