Ruby 2.2 will define Pathname#/ as a simple alias of Pathname#+.
In practice, this means that it will raise a TypeError unless the
argument responds to to_path or to_str.
Currently we blindly convert the argument to a string using to_s, so
deprecate this in the interest of matching the upstream behavior. In the
future we can replace this with
alias_method :/, :+ unless method_defined?(:/)
ClosesHomebrew/homebrew#30079.