Ordering api deprecations

This fixes #1092.  In that PR we discussed adding a warning for when `:order` metadata is used since it is changing meaning in RSpec 3 (now that rspec will use it).  However, I decided against adding that. It feels like overkill.  Warning in this case is a bit like issueing a warning if we're going to introduce a new `foo` matcher and users have defined a `foo` helper method in their example groups.  The meaning of `foo` will change now that RSpec will use it...but we don't issue warnings for such cases because it's overkill and RSpec is free to add new things when it needs to.

/cc @xaviershay @JonRowe