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.
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).
Phantomjs: Yosemite fix

Removes the unnecessary head dependency.
phantomjs: update 1.9.7_1 bottle.