Deprecate Pathname#/ with non-string/non-pathname arguments

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

```rb
alias_method :/, :+ unless method_defined?(:/)
```